Correction - you
were a lurker.
Welcome!
Don't worry about it - I have only foggy notions of the "legacy" and "challenge" stuff, myself. And that's fine - everyone plays the game the way they like. These challenges are just ideas some users have shared for giving a very open game some structure, or a competitive element.
I -like- my game without structure, and much prefer to collaborate than compete. The challenges are there if you are interested. If it's not your thing, then yeah, a lot of the discussion may seem opaque. I just mentally gloss over it as "this thing I am doing in my game with self-imposed rules".
But plenty of discussion here of all aspects of play, and friendly folks. And all the tons of solid Sims 3 guidance that Carl's serves up so nicely for us, of course.