Definitions to these tend to be specific to the company you work for.
Generally these are the definition that have a common thread through different companies that I have worked at:
Maintenance Release - contains corrections/fixes to a current production system
Enhancement Release - contains enhancements/improvements/additional features to a current production system
Major Release - a new product release or a release that contains a considerable enhancement to a current application. Definition of major can be tied to the functionality and/or the time it takes to produce the Release.
Minor Release - small new product, small enhancement release or a maintenance release. Definition of Minor can be tied to the functionality and/or the time it takes.
I have not failed. I've just found 10,000 ways that won't work" --Thomas Edison