RC stands for release candidate - most everything is fixed although they are still working out bugs - there will be no more major changes once RC1 is released. Things may get pulled if they can't fix it before RC3, which will go live roughly 2 months after RC1, then comes RTM - Release to Manufacturing, meaning OEM's get it, and that is roughly 1-2 weeks before they can ship. ALthough there is always some company that will ship within one week of RTM so that they can suck up sales as being the first releaser.
If RC2 is out, then there are probably less than 20 bugs that still need to be tackled, none of them sever enough to stop ship, but bad enough that they would cause endless headaches and calls to OEM's and suck up potential profits. RC3 shoud hit approximately 1 month from now, so middle of March, and that will pretty much be shipping version, with minor tweaks and such to do, major headaches will have been pulled out and lined up for a service pack release around a year from RTM, which would probably be start of April, and ship about mid April right around the time of earnings calls for OEMS, MS, Intel and of course AMD. Big announcement time, lots of nice forward looking projections, blah blah blah.
RC builds still are betas - but most people have no access to builds after RC1, unless you sign up with MS as a developer then you can get access to them (along with signing an NDA). RTM is considered a gold product which means there will be no more changes to the code before shipping and it gives a window for OEM's to get their manufactuing process finalized for shipping the product.