Okay, well up until a few months ago they had two products, the iBook and the Powerbook.
iBook = consumer model, 1.3-1.42GHz G4 processor, $999-1299, 12" or 14" screen, white plastic case
Powerbook = professional model, 1.5GHz+ G4, $1499 and up, 12", 15", or 17" screen; silver aluminum case
Under the skin, there really wasn't a huge difference. The Powerbooks are had faster processors and better video cards and bigger hard drives, but that's about it.
Things got all messed up back in January when they introduced the MacBook Pro, which is replacing the Powebook lineup. It has an Intel Core Duo processor, which is (supposedly) faster than the G4 found in the Powerbooks and iBooks.
They're still selling the iBook, but its days are numbered. They WILL be introducing a new Intel-powered iBook replacement sometime soon, most likely called the MacBook (no "Pro"). Despite what others are saying, I can pretty much guarantee that the iBook/MacBook will NOT have an Intel Core Duo processor -- it will be a single-core Intel chip. Other rumors point to a single 13" widescreen model, replacing both the 12" and 14" models. Also, it's very likely that the MacBook will have integrated Intel graphics with shared memory -- a drawback, in my opinion, considering that the iBooks have an ATI Radeon 9550 with 32MB of dedicated video memory.
In any case, Apple laptops are great... I myself have a 14" iBook G4 which does everything I need it to and then some. If you can find a bargain on a G4-powered Powerbook or iBook, go for it... just be aware that Apple is phasing those products out, and Intel is the future for Apple (unfortunately... I'm not a very big Intel fan. Go AMD!)