Underdrive pullies mean they will "drive" whatever systems are attached to it less than the current one, ie the pullie will be smaller and each revolution of it will spin the attached pullies less. I think they are used on cars which rev more than stock and thus when they get up the rev range, dont overdrive the attached other bits and bobs (alternator, AC, etc). Thats my take on it, someone more knowledgable will be along to correct me if wrong
Basically unless you know you want an underdrive one, you want a standard size.