CMMI v1.3 Process Areas
https://www.benlinders.com/tools/cmmi-v1-3-process-areas/ – This link contains information on the process areas and the cmmi level that they represent.
Category: CMMI
Intro to CMMI Levels
This link has good introduction to CMMI Levels – https://msdn.microsoft.com/cs-cz/library/ee461556(v=vs.100).aspx