Honor is a vague term referring to the building of honor points via PvP to gain rewards.
See Honor System for more details.
Prior to the Patch 2.0, it was also the label for the tab on the player character's paper doll window that displayed the honor earned by the character; after the patch this tab was renamed "PvP". However, the tab still retains its "H" key shortcut to display.
See also
- Patch 2.0 Honor System Changes at the official site
- WotLK Honor Update (September) at the official WoW forums.