well, it's impossible to write something like an OS all at once, so they will write some bits, then turn the code into a working version of the program called a build. they test the build, find problems or things that need to be tweaked. they'll make those changes to the code, then make another build.
essentially, builds are test versions of the real thing. these sometimes leak out to the internet and people like us download them and try them out.
officially, microsoft has only released 2 builds - the Beta, and the Release Candidate. the beta build is usally the first build that's good enough for the public to try out. the Release Candidate is something inbetween the Beta and the Final build.
we're getting close to the final bulild now. it's usually referred to as RTM, which means Release To Manufacturing - it's the version that microsoft sends out to companies like dell and hp so they can start installing it on hard drives to be sold with new computers starting in late october.