Options and cost.

A Macbook is a Macbook. There's one model, with a 13" screen. You can customize it with a bigger hard drive or more memory, but that's about it. 2 USB 2.0, 1 Firewire 400 port.
http://store.apple.com/us/browse/home/shop_mac/family/macbook?mco=MTE4MTY

There are several models of Macbook Pro, some with more options to customize them than others.
http://store.apple.com/us/browse/home/shop_mac/family/macbook_pro?mco=MTE4NDY

The MBP 13" has a thinner, aluminum chassis, has Firewire 800, a slightly faster processor, backlit keyboard, and an SD slot.

Then, there are MBP 15.4" and 17" models. Faster processors, more RAM, more HD space, higher screen resolutions, and more options to customize.

M80v2 | VP150v2 | QS8v2
SVS Pci+ 20-39
Emotiva UMC-1 & LPA-1
M22ti + T-Amp, in the Office