Why are there two different sleep states on a MacBook?
There are really three, sort of, but they're not exactly as you described:


Display sleep: The screen goes blank after a certain amount of time. You can wake up the screen by touching the trackpad or pressing a key.
Sleep: In this state, the computer goes to sleep: the hard drive stops spinning and the CPU uses much less (almost no) power. This can be invoked by closing the lid of the laptop, selecting Sleep from the Apple menu, or via a timer. You wake up the computer by opening the lid or pressing a key.
Hibernation: If the battery is almost dead, the laptop will hibernate to save power. This is similar to shutting down, except all the contents of RAM are dumped to the hard drive, which makes it much faster to start up (as well as starting the computer up into the previous state). You have to hit the laptop power key to wake the machine from hibernation.