STM is an acronym for Stepper Motor. USM is an acronym for Ultrasonic Motor. The specific STM and USM nomenclature is strictly Canon, but other lens makers have similar terms for the USM concept, as well as newer linear types of AF motors. Canon dubbed their new linear AF motor Nano USM in the new EF 18-135mm f/3.5-5.6 IS USM and EF 70-300mm f/4-5.6 IS II USM lenses, but Nano isn't included by Canon in the official lens names.
Basically the names are for designating different types of motors that convert electricity into mechanical movement in order to focus the lens.
For a good comparison of how STM and USM lenses work please see this answer to What does STM mean on a Canon lens?.
For how those differences work out practically, please see: Why do higher end lenses use USM instead of STM? and STM vs Non STM lenses for still photography??
To the best of my knowledge, Canon is the only lens maker currently using stepper motor technology for AF in some of their lenses designed for interchangeable lens cameras.
Many makers use variations of USM. Nikon calls it Silent Wave (SWV), Sony calls it Super-Sonic Motor (SSM), Pentax calls it Supersonic Drive Motor (SDM), Olympus calls it Supersonic Wave Drive Motor (SWD), Sigma calls it Hyper-Sonic Motor (HSM), and Tamron calls it Ultrasonic Silent Drive (USD).
Linear motors, with a variety of different names, are becoming quite common in lenses for newer Micro 4/3 and other mirrorless camera systems.