UPDATE: The cheat sheet linked is more than likely out of date, so it would be best to head on over to the Esri/arcpy GitHub repo for the latest resources (and updated cheat sheet). Happy coding!
ArcPy (often referred to as the ArcPy site package) provides Python access for all geoprocessing tools, including extensions, as well as a wide variety of useful functions and classes for workign with and interrogating GIS data. Using Python and ArcPy, you can develop an infinite number of useful programs that operate on geographic data. Here is a cheat sheet you can use for more information.
