The VIP fastpass allows you to enter the fastpass queue. The fastpass queue is normally less than 10mins long.
Fastpass rides:
Big Thunder mountain,
Peter Pan,
Space Mountain,
Star Tours,
Buzz Lightyear,
Indiana Jones,
Alladin,
Crush's Coaster (temp fast pass)
Rock N Rollercoaster.
Everyone who enters the park is entitled to a standard fastpass. Standard fastpass' require you to put your park ticket into a fastpass machine (located next to rides with fastpass). You will then be given a little ticket telling you to join the fastpass queue at a specific time. Standard tickets are only allowed one fastpass at a time (so you have to wait for the printed time to expire to get a new one)
The benefits of VIP fastpass are obvious, you wont have to queue for so long. As to whether it's worth it; peak season = yes, off-peak = no. The waiting times (even for the popular rides) are short off-peak. In the summer some queue's are 2 hours long, in the winter (excluding hallowean & xmas) the most you'll wait is 30mins.