iLife and iWork are separate from the Mac OS. There are some very old versions of iLife that aren't compatible with Leopard, let alone Snow Leopard, but you'd have to have a PowerPC Mac to have one of those versions. So, in other words, no, you won't also have to upgrade iLife and iWork, too, despite Apple's attempt to sell you the bundle.

For someone upgrading from Leopard to Snow Leopard, you get a lot of little but mostly under-the-hood improvements and performance boosts. However, going all the way from Tiger, you get all the major features that Leopard added, most notably Time Machine, but also Spaces.

I could go on and on, but I think that at this point it would be best to ask if there are any specific things you'd like me to talk about.