Defending is defined by forcing the enemy team to give up, disband, dissolve, etc.. and go home. (To whine in trade because they lost!)
Capturing Halaa is just that, capture the town! Kill all the guards and capture the flag in the middle of the town to win!
(If you don't know what Halaa is, I suggest you read "wowwiki.com/Halaa")
The Defenders/Attackers will of course be determined by what ever faction holds Halaa by 7:00 Feb 29th.
If you need a calendar event to remind you of this awesome event then send a whisper or in-game mail to Donnathadead for Horde or Kluuvdar for Alliance.