BETA is a testing phase. it comes after a private, in-house alpha (usually).
During the beta phase, developers get feedback on the product, and they subsequently revise their design documents and so forth.
After the beta phase- it goes into "release Candidate" where the design documents are locked. So although the RC is basically another Beta it is close to what will be released. Changes will usually be limited to show-stopping bugs or major issues.
Patio says to not use it as the main OS because- either way- Beta or RC- it's a "test" operating system- it will have more issues all-around then a released OS, such as Vista, or XP, not to mention the time restrictions that will come into effect.
