The Combat Log is a log of combat actions and results, but may also show the results of the profession activities of nearby characters.
By default, it is accessible via a Combat Log tab on the General chat window.
Combat Logs may be written to a file using the /combatlog command. The resulting file may be found at Logs\WoWCombatLog.txt. The complexity of the combat logs content since the patch 2.4 changes means that logs are best used when analyzed by one of the available parsers which analyze combat logs to generate raid statistics.
The first such parser of wide popularity was Wow Web Stats. Other emergent parsers include WowMeterOnline, World of Logs, MMOStories all of which demonstrate compatibility with logs from patch 2.4 and beyond.
Patch 4.0.6 (8-Feb-2011): The Combat Log was displaying a lot of incorrect or inconsistent information pertaining to select player, creature, and boss abilities. These issues have been addressed.
/Patch 4.0.1 (12-Oct-2010): Detailed combat log tooltips that use tokens are scaling based on the players’ spellpower
/Patch 3.0.2 (14-Oct-2008): Changes for Wrath of the Lich King:
The combat log now differentiates between a spell failure due to resistance and spell failure due to missing the target. Where once both events reported as a resist; a spell missing the target is now reported as a miss.
Overhealing is now reported in the combat log.
When a source of damage is entirely prevented (by a shield block, a full resist, or a damage shield like Power Word:Shield, the prevented amount will now be displayed.
New tabs are available for sorting combat messages.
ctrl-right clicking functionality has been added for individual information sorting
Colors may now be assigned for friends and enemies
The WoWCombatLog.txt file format has been changed to make it easier for programs to read.
The ability to chat-link player spells and talents.
Patch 1.8.0 (10-Oct-2005): When you gain reputation, the combat log message will now display the numeric value of how much reputation you gained as well as the previous relative message.