Maya comes packaged with the following libraries:
maya.cmds: Python wrapper for MEL commands that can be used in place of MEL. For more information, see Using Python and the Python command reference.
pymel: PyMEL is a third party wrapper for MEL commands and is not supported by Autodesk. PyMEL can be installed as a Maya component, or it can be installed using pip.
maya.OpenMaya: Referred to as Python API 1.0. It is used for developing Maya plug-ins and applications. For more information about Python API 1.0, see Maya Python API 1.0 in the Maya Developer Help.
maya.api.OpenMaya: Referred to as Python API 2.0. It is also used for developing Maya plug-ins and applications. It is more pythonic but less comprehensive than Python API 1.0. For more information, see Maya Python API 2.0 in the Maya Developer Help and the Maya Python API 2.0 Reference.