It is an interesting experience... there are a few ups and downs to it and a few things you should take into consideration when using it.
It is a 3-D Avatar Chat program, where many members make items (those members being called developers and the process of making the items called developing) for the avatars, and the members buy them with Credits (which can be bought various ways: directly from IMVU or directly from a really good developer, or even from an associate group of developers) or Promo Credits (free credits that can only be used for yourself, only retrieved from doing various tasks on IMVU).
This site is (supposedly) only comprised of people ages 13 and above. There are Adult users that hold special passes to make certain items and actions (during chat) available to be seen by other Access Pass holders. If you have any 'specific' questions dealing with IMVU, I'll be happy to help, and if I can't answer it, I know there are others out there that can answer them for you.