Wikipedia's definition is quite good, but is a little too long to repost here: http://en.wikipedia.org/wiki/Digital_rights_management
In any event, we're talking about technical solutions to licensing problems.
I haven't read any reports of Google or Amazon's cloud music players adding DRM to files after they've been uploaded. Has anyone heard differently? I thought the whole reason they were legally in the clear is because they're basically file lockers with playback functionality.
The "Digital Audio Recording Devices and Media" amendment to the AHRA may be an example of the Recording Industry's lobbying efforts being rewarded more than we'd like, but DRM it is not.
As for your warning letter from the RIAA: did they have reason to believe you wouldn't be paying your publishing royalties? I imagine your record label probably works with the Harry Fox Agency all the time. Was there something that was unusual about your case?