Blizzard has recently (within the last 2 weeks) released a new edition to Starcraft II.
This new edition allows anyone to download the full Starcraft II client and be able to play online with their account, even if they don't own the game.
When you logon with an account that does not have the game, Battle.net will automatically give you a Starter Edition license, and you will be able to play online for unlimited amounts of time, but only up to certain scenarios/levels and you will be without chat channels.
Of course, you can always logon to your account through the Battle.net website and get yourself a Starter Edition license from there, but you do not have to. If you do not currently own the game or do not have the full client, you should do this anyway so that you can get the download from Battle.net's online download system.
As always, enjoy the website! :)