The creature takes a -2 penalty to attack rolls for any attack that doesn’t include the marking creature as a target.
A creature can be subjected to only one mark at a time, and a new mark supersedes an old one.
A mark ends immediately when its creator dies or falls unconscious.
This condition reflects the ability of some creatures to claim the attention of a chosen target in battle. When a target is marked, it has a hard time ignoring the creature that marked it. Most marking effects have very short durations, or else they require the marking creature to remain a threat to the marked target.
Published in Player's Handbook, page(s) 277, Monster Manual 2, page(s) 218, Player's Handbook 3, page(s) 222, Rules Compendium, page(s) 232, Monster Vault: Threats to the Nentir Vale, page(s) 124.