There is probably some official page somewhere that defines these terms better. But since I toss them around around all the time, I probably owe it to some readers to explain what they mean. Here goes, from least to greatest:
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 shrinkwrapped
To top it off in a way that is likely to cause me future embarrassment, I will treat you to a bad car wash analogy. CTP quality assurance is like driving through the self-serve car wash, or maybe like having it rain so often that a car wash is not necessary. One of the perks of living in Washington state. Beta release quality assurance is like driving through the self-serve car wash and splurging for self-serve vacuuming (during the beta stage you really do have a chance to participate in the process and influence quality). RTM quality assurance is the deluxe full service car wash. :)