Maya is a 3D program, which can be used to make the 3D shapes of objects for The Sims 2. It's a commercial product, but Maxis will probably release a trimmed-down version of it for the Sims 2. You can already try out maya already by checking out
its website.
CaSIE is used to create skins and clothing for The Sims 2. It's not out yet and Maxis hasn't told when exactly it will be yet. Also check out the separate "CaSIE" category in this knowledge database.