I am curious, what do EF, EF-S, and USM stand for?

Thanks.

I am curious, what do EF, EF-S, and USM stand for?

Thanks.

These are all terms for Canon products.

EF = Electronic Focus. It is the name for the type of "mount" used on the lenses and camera.
EF-S = I have no idea with the "S" stands for but it's referring to a newer type of lens mount that can be found on the EOS 20D and Digital Rebel camera. In a nutshell, this mount allows for wider angle lenses.

USM = Ultrasonic Motors. It's a kind of motor that Canon puts into some of it's lenses. The motor is used for focusing. Typically, USM is quieter and quicker than non-USM but this may not always be the case.

I believe that the S in EF-S stands for short. The back part of the lens is shorter than the typical EF lens and is designed to work with a specific camera body. For example: the EF-S 18-55 lens that comes with the DRebel kit only works with the DRebel.

EF is the Canon lens mount in use today, older FD lenses don't fit an EF mount camera.

EF-S is for short backfocus lenses.
That's where the rearmost element of the lens protrudes into the camera body, and would be hit by the mirror of an EF camera. EF-S is only found (today) on the digital cameras.
It's used by the 10-22mm lens which fits the 20D and 10D - but not the 1D.

USM is the focussing motor, UltraSonic Motor , used in the lens. It's allegedly faster and quieter focussing than other lenses - but al my lenses are USM so I can't compare.

10-22mm
18-55mm
17-85mm IS

The current cameras that take EF-S lenses are the EOS 20D and the 300D/DRebel

The 10D does not take the EF-S lenses, although some people did modify the 18-55 lens so it would fit the 10D

The incorporation of the EF-S mount was one of the significant changes between the 10D & 20D