15 amp vs. 20 amp: please explain

I'm not really sure what the difference is. I see power cords and power conditioners that are designed for one or the other. Is one "better" than the other? Thanks.
All other things being equal, the only difference is the current-carrying ability. Better depends on other things not being equal.

In addition to current-carrying ability, 15 amp power cables have a different female end on them than a 20 amp power cable.

These are not compatable with each other. Therefore, you must know which is the correct one for your gear.

A good majority of the equipment out there use a 15 amp IEC connection. 20 amp are more common on large amplifiers, etc.