The first use of the term is Breishis 2:4
ד. אֵלֶּה תוֹלְדוֹת הַשָּׁמַיִם וְהָאָרֶץ בְּהִבָּרְאָם בְּיוֹם עֲשׂוֹת יְהֹוָה אֱלֹהִים אֶרֶץ וְשָׁמָיִם
4. These are the generations of the heavens and the earth when they were created, on the day that the Lord God made earth and heaven.This is the end of the "era" of creation and the beginning of the "era" of existence.Before this everything came into existence with an explicit דיבור (statement) as specified in the seven "days" of creation. That "era" ended with the creation of שבת (Shabbos) which was the explicit command to the universe to stop creation and begin the process of "nature". This continues through the story of Adam and shows how humanity failed the test of the garden. The story of Cain and Abel is part of this story and the generations of Cain's line is considered part of it. That is because Cain and his line cannot be considered a new "era", but just follow along in nature. It is only with the birth of Adam's third son Seth that we can consider the era of Adam to begin. Cain and his line continue in parallel but do not create a significant era..
Bereishis 5:1 begins the next story arc of significance.
א. זֶה סֵפֶר תּוֹלְדֹת אָדָם בְּיוֹם בְּרֹא אֱלֹהִים אָדָם בִּדְמוּת אֱלֹהִים עָשָׂה אֹתוֹ:
1. This is the narrative of the generations of man; on the day that God created man, in the likeness of God He created him.After the era of creation, we come into the era of Nature as symbolized by Adam. This era continues with humanity attempting to live as a "natural being" and failing. It is here that we actually are given the history of humanity with the ages of each member of Adam's line at the birth of the eldest son in that line. We should notice that, while the names of Cain's line is given, none of the ages are given so that we cannot connect them to the actual history of the world. On the other hand, we are given the exact timeline of Seth's line so that we see how the history continues. Rabbi Samson Rafael Hirsch points out that each of the names given is actually significant of the characteristic of that generation. While Cain's line is actually monotonically decreasing in spiritual level, Seth's line will decline and then improve before declining again. Both lines end with "Lemech". Rav Hirsch says that the meaning of this is etymologicaslly unknown. However in Cain's line his father is "מתושאל (Mesushael) which implies "death seeking masses of people". Seth's Lemech on the other hand is fathered by מתושלח (Mesushalech) which implies that the masses were מתים (dead) and were abandoned (שלח).
The children of Cain's Lemech were Yaval, who is described as the father of business, trade, and merchandising. Yuval who was the father of art and esthetics. he no longer produces, but his name is in the passive sense. An artitss in effect lives off the producers of society who support them because of the pleasure that they bring. Tuval Cain is the father of science and research. He is described as "sharpened everything which cuts copper and iron". He does not produce anything dirctly, but instead makes it possible for the other members of society to produce. His sister is Naamah, who the medrash says bacame the wife of Noach. She alone of the entire line merits saving by separating herself from the rest of her family.
The Lemech of Seth's line gave birth to Noach who the medrash says brought rest to the world and is the actual start of the next era. An interesting sidelight is that מתושלח, who dies seven days before the flood, outlives his son למך נט by 5 years. THus, every member of Seth's line who is specifically mentioned (except for Noach and his sons) has died before the flood begins.
Of course, the next era starts with Noach in Parshas Noach 6:9
ט. אֵלֶּה תּוֹלְדֹת נֹחַ נֹחַ אִישׁ צַדִּיק תָּמִים הָיָה בְּדֹרֹתָיו אֶת הָאֱלֹהִים הִתְהַלֶּךְ נֹחַ:
9. These are the generations of Noah, Noah was a righteous man he was perfect in his generations; Noah walked with God.
This "era" ends the existence of the natural world and returns the earth to the phase of creation. The existence of humanity is back to an explicit miracle with survival in the ark being possible only because the "laws of nature" have been suspended. It is obviously impossible under the laws of nature for a human being to build a vessel and be able to bring samples of every animal as well as a year's worth of food into it. It would also be impossible for many of the insects to survive the entire period of the flood and it would have been impossible to replenish the earth since all living things had been destroyed except for what was in the ark. In effect, the era of Noach is a recreation of the world.
Noach is a transitional figure. He brings humanity out of the original world, through the new creation and into the new world which must be rebuilt. Noach 10:1 gives us the next part.
א. וְאֵלֶּה תּוֹלְדֹת בְּנֵי נֹחַ שֵׁם חָם וָיָפֶת וַיִּוָּלְדוּ לָהֶם בָּנִים אַחַר הַמַּבּוּל:
1. And these are the generations of the sons of Noah: Shem, Ham, and Japheth, and sons were born to them after the Flood.Here we have an improvement over Adam as all three sons prove capable of continuing humanity in their lines. They do fail in a respect and suffer the punishment of the dispersion. However, they are not destroyed. The descendants of Yaphet and Cham continue in the way that their nature provides and are no longer relevant to the story. However, Shem begins his own era and his lineage is treated in the similarly to that of Seth. See Difference between תולדות אדם and תולדות שם for a discussion of the difference between them.
At this point the narrative narrows down to follow the lineage of Shem who continues following the way of G0d.
י. אֵלֶּה תּוֹלְדֹת שֵׁם שֵׁם בֶּן מְאַת שָׁנָה וַיּוֹלֶד אֶת אַרְפַּכְשָׁד שְׁנָתַיִם אַחַר הַמַּבּוּל:
10. These are the generations of Shem: Shem was one hundred years old, and he begot Arpachshad, two years after the Flood.Notice that Breishis 27:32 says that Noach gave birth to Shem Cham and Yaphet 100 years before the flood, but that Shem was 100 years old two years after the flood. Either the original pasuk means that Noach started having children then and Shem was the middle child, or time did not pass during the "era" of the flood.
Just as Noach and his children restarted humanity after the destruction of the flood, Shem and his children restarted spiritual humanity. This lineage continues to Terach who, just like Noach, has three sons. and who, just like Noach survives the punishment that is brought to the world in order to continue the line of spiritual humanity. It is Terach who is treated as the end of the transitional era represented by Shem. Humanity has survived, flourished, and replenished the earth. It is now time for the reversal of teh spiritual decline to occur. It is interesting that Avraham does not get referenced as the start of the era. Even though Terach is still part of the idol worshipping world, it is he who will be regarded as the end of the 2000 years of chaos and the start of the era symbolized by the existence of Avraham. Avraham himself is not regarded as a "separate" era and the term Toldos is not used with respect to him.
He has two sons, Yishmael and Yitzchak. They are regarded as two separate streams of humanity and will therefore be considered the branching point. That is why we have Lech Lecha 25:12 and Toldos 25:19.
יב. וְאֵלֶּה תֹּלְדֹת יִשְׁמָעֵאל בֶּן אַבְרָהָם אֲשֶׁר יָלְדָה הָגָר הַמִּצְרִית שִׁפְחַת שָׂרָה לְאַבְרָהָם:
:12. Now these are the generations of Ishmael the son of Abraham, whom Hagar the Egyptian, the maidservant of Sarah, bore to Abraham.
יט. וְאֵלֶּה תּוֹלְדֹת יִצְחָק בֶּן אַבְרָהָם אַבְרָהָם הוֹלִיד אֶת יִצְחָק:
19. And these are the generations of Isaac the son of Abraham; Abraham begot Isaac.Each one starts a completely different line of history. Just as the Torah drops the sons of Noach to follow Shem, and the children of Shem to follow Terach, we now end the summary of Yishmael with the indication that his descendants have their own history and follow Yitzchak. Avraham was an individual. Yitzchak and Yishmael create families that will become nations. It is Yitzchak who is regarded as the true heir of Avraham and who we will follow.
Just as Noach was a transitional figure, Yitzchak too is a transitional figure who changes the spiritual heritage of Avraham from that of a unique human being and teacher into that of a family. He is the first person who is actually born into the spiritual era of humanit.His legacy again leads to a split that is shown by the use of Toldos referring to each of his sons, Yaakov and Eisav become the progenitors of their own nations and end the use of Toldos. Eisav, like Yishmael, branches off creating his own line and leaving the history that the Torah is teaching us. Yaakov will create the 12 shvatim which will become the nation of Yisrael. We see the reference to Eisav in Vayishlach 36:1 and the reference to Yaakov in Vayeishev 37:2
א. וְאֵלֶּה תֹּלְדוֹת עֵשָׂו הוּא אֱדוֹם:
1. And these are the generations of Esau, that is, Edom.
ב. אֵלֶּה | תֹּלְדוֹת יַעֲקֹב יוֹסֵף בֶּן שְׁבַע עֶשְׂרֵה שָׁנָה הָיָה רֹעֶה אֶת אֶחָיו בַּצֹּאן וְהוּא נַעַר אֶת בְּנֵי בִלְהָה וְאֶת בְּנֵי זִלְפָּה נְשֵׁי אָבִיו וַיָּבֵא יוֹסֵף אֶת דִּבָּתָם רָעָה אֶל אֲבִיהֶם:
2. These are the generations of Jacob: when Joseph was seventeen years old, being a shepherd, he was with his brothers with the flocks, and he was a lad, [and was] with the sons of Bilhah and with the sons of Zilpah, his father's wives; and Joseph brought evil tales about them to their father.From this point on we see the continuation of Bnai Yisroel and the "Toldos phrase" denoting the end of an era and the beginning of a new one no longer occurs. From now on our history is an uniterrupted lin from the descent into Egypt, the famine, the Exodus, the receiving of the Torah, the entry into canaan, the creation of the monarchy, the building and the destruction of the Temples, the exile, the trials and tribulations that have led to our day.
Had Hashem destroyed us at the Golden Calf and restarted the people with Moshe as he had threated, we would have had anothe "Toldos" moment. However, thanks to Moshe Rabbeinu, Hashem held back and allowed us to survive. Let us hope that we finally learn the lesson. This history is an unbroken line that will lead to the coming of the Moshiach, "speedily and in our day".
The final usage is in Bamidbar 3:1 and can actually be considered part of this analysis.
א. וְאֵלֶּה תּוֹלְדֹת אַהֲרֹן וּמשֶׁה בְּיוֹם דִּבֶּר יְהוָֹה אֶת משֶׁה בְּהַר סִינָי:
1. These are the descendants of Moses and Aaron on the day that the Lord spoke to Moses at Mount Sinai.This follows the full census of Bnai Yisrael and changes the nature of the people from a simple unity, with the bechorim (first born) being set aside to serve Hashem, to a nation with the priestly "caste" and the Levites consecrated from birth to serve Hashem. Rashi points out
ואלה תולדת אהרן ומשה: ואינו מזכיר אלא בני אהרן. ונקראו תולדות משה, לפי שלמדן תורה. מלמד שכל המלמד את בן חבירו תורה, מעלה עליו הכתוב כאלו ילדו:
These are the descendants of Moses and Aaron: Yet only the sons of Aaron are mentioned. However, they are considered descendants of Moses because he taught them Torah. This teaches us that whoever teaches Torah to the son of his fellow man, Scripture regards it as if he had begotten him - [Sanh. 19b]
ביום דבר ה' את משה: נעשו אלו התולדות שלו, שלמדן מה שלמד מפי הגבורה:
on the day that the Lord spoke to Moses: they became his descendants, because he taught them what he had learned from the Almighty.The separation of the Levites follows and once this is done, the nation of Israel has been established. From now on we have an actual nation with everyone given a role to play and a responsibility. As Rabbi Sacks points oout in his drasha on Parshas Terumah, up until the building of the Mishkan, the Bnei Yisrael were passive and acted upon. It is with the Terumah and the building of the Mishkan that they had to stand up and take responsibility for themselves and act on their own. This is what makes a people and this is what enables them to continue.