Hi and welcome to the forums, Kindofagirl! I am glad you are enjoying The Sims 3. Generations is great, believe me, you'll enjoy it.
Oh, dynasties and legacies. Well, those are challenges written for Sims 3 players. A dynasty is a save file for the
Immortal Dynasty Challenge, written by our Metropolis Man here, a legacy is a save file for the
Legacy Challenge, written by Pinstar, on the official forums.
The main goal in a dynasty is to obtain a family of eight generations of immortals living under one roof. There are a lot of rules, including many requirements in order to become truly immortal.
The main goal in a legacy is to have a multi (10) generational family, and explore the game in depth with it. Your legacy family is a household that you'll have to stick with for a year or more, so be sure to enjoy them. Some players, like me, get bored of legacies pretty quickly and start a new one. I do hope my current attempt succeeds, lol.
Best of luck with your dynasty/legacy, if you do one of course.