The regular API is part of the CoppeliaSim API framework.
The regular API is composed by several hundreds of functions that can be called from a C/C++ application (plugin or main client application), or an embedded script. CoppeliaSim functions and constants can easily be recognized from their "sim"- or "_sim"-prefix (e.g. sim.handleCollision). Make sure not to mix-up the regular API (sometimes also "API" in short) with the remote API.
The regular API can be extended by custom lua functions, that plugins or the main client application register. Custom lua functions can be recognized from their sim*.-prefix.
All units going to, or coming from the API are in meters, kilograms, seconds and radians or a combination of those (unless otherwise explicitly indicated). The user interface units are in meters, kilograms, seconds and degrees.