alright it seems kiddy at first because its a lego, but its for ages 7-16. What you do is:

You can build your own levels
Visit other friend's levels
Chat (Two modes, SuperSafe and Safe)
Buy and wear clothes
Talk on forums (Yes there's forums xD)

And to buy stuff, you need to earn Tickets or Robux. If you have an account, you get 10 Tickets every 24 hours. But you need to be a member (Called 'builders club') to get 20 Robux every 24 hours. and then you can use your Tx/R$ to buy clothes, and lots of other stuff!

I suggest you play as a guest first to see if you like it. It takes a while to get used to it.

DICTIONARY:
Obby: Obstacle Course
Cloths: Clothes
Sparta: (I have absoloutly no idea what Sparta means lol)
builderman: The main admin
ROBLOX(1): The game
ROBLOX(2): Creator of ROBLOX.
BC: Builder's Club

well that's all you need to know! Post on here if you like it, hate it, have questions, etc.

HAVE FUN!!

http://www.roblox.com/

P.S: you need to pay for builders club

I downloaded this a while back and couldn't understand how it worked. Maybe my graphics card isn't up to date, but I seemed to be going in slo-mo, while the people creating stuff around me were building amazing structures in super speed!

I was about to play it, then when I realised I had to download something, I couldn't be bothered anymore, too late to be messing around with stuff.

lol