[ChrisG]

Quoted from Wiki:

Variants 1 and 2 (double action/single action, decocking and safety lever)
Variants 1 (lever on left) and 2 (lever on right) allow the user to carry the pistol in a single-action mode (cocked and locked) with the manual safety engaged. This same pistol, without modification, can be carried in double-action mode, with or without the manual safety engaged, and with the benefit of a decock lever.

Variants 3 and 4 (double action/single action, decocking lever, but no safety)
Variants 3 (lever on left) and 4 (lever on right) provide the user with a frame-mounted decocking lever that does not have the "safe" position. This combination only allows the hammer to be lowered from SA position to DA position. It does not provide the "safe" position to prevent the pistol from firing when the trigger is pulled.

Variants 5 and 6 (double action only, with safety lever)
For the double action only user, variants 5 (lever on left), 6 (lever on right), and of the USP operate as double action only pistols with a bobbed hammer always returning to the DA position (forward) after each shot is fired. To fire each shot, the trigger must be pulled through the smooth DA trigger pull. Variants 5 and 6 have a manual safety lever.

Variants 7 (double action only, no control lever)
No control lever is provided on variant 7.

Variants 8 and 9 (double action/single action, safety lever, but no decocking)
Variants 8 (lever on left) and 9 (lever on right) allow the shooter to carry the pistol in a single-action mode (cocked and locked) with the manual safety engaged. This same pistol, without modification, can be carried in double-action mode (hammer down), with or without the manual safety engaged. The double action mode offers a second strike/double action capability in case of a misfire. The control lever has no decocking function on variants 8 and 9 so one would have to carefully lower the hammer manually.

LEM (Law Enforcement Modification) Trigger
Developed especially for the U.S. government, the Law Enforcement Modification (or LEM for short), is a USP DAO (Double-Action Only) model with a unique trigger mechanism. This mechanism improves the double action trigger performance and reduces the weight of the DAO trigger pull to 7.5-8.5 pounds. Aside from the LEM mechanism, it is identical to the USP Variant 7 (DAO) model.