CTP - community technology preview - this is just a point in time release to get more bits into hands of customers between beta releases, for those hard core people who want to be as close to the action as possible. 
Beta - these releases get special attention from the QA Team, we plan for them, treat as a milestone, etc.
RC - release candidate, this is the one we hope will become an RTM.  Testing additional RC candidates can get to be like a baseball game in extra innings.  When we finally say "ship it" everyone celebrates.
RTM - release to manufacture, this is the RC that gets shrink wrapped.