Team Fortress 2 is ever-evolving and a new initiative from Valve hopes to make this rampant evolution a bit safer for all players through the Team Fortress 2 public beta. Accessible to all owners of the game on Steam (open your Library then install "Team Fortress 2 Beta" from your games list), it's a special version of the game that lets Valve tinker around with game mechanics while you test 'em out live. Think: scientist and guinea pig. You're the guinea pig.
Valve has detailed the first few changes in the beta right now and published a brief FAQ -- if you've still got questions, hit up the link below.