From TF2 Wiki
This is a complete listing of all Achievements available in Team Fortress 2. There are currently 381 total achievements (353 excluding Milestones). For all statistics relating to achievements, see on the Steam website.
Achievements are in-game enhancements that allow players to be rewarded for special actions while playing. There is currently a set of General Achievements and a set for every class, in addition to the special Halloween set.
When achievements are obtained, a trophy image appears briefly above the player's head, dropping pink and purple streamers, and a party sound will play with your achievement being announced to all players in the server. However, when playing as a Spy the achievement will only be announced to your team. Some achievements bring rewards for players who unlock them. When a player unlocks a certain amount of achievements in a Class set, they also unlock a Milestone achievement, which will reward them with an unlockable weapon. The weapon unlocked will be labeled as "achievment non-tradeable".
All achievements can be completed at any time in the game, with the exception of a few from the Halloween sets which could only be obtained during the Halloween Special in 2009 and Scream Fortress Update in 2010.
Class Specific Achievements
There are 18 general achievements. These achievements were available since the game's release, before the class updates added more class specific achievements. For tips and strategies on how to complete these achievements, refer to the Obtaining Achievements page. Class-specific general achievements do not count towards achievement milestones for that class (e.g. Earning the Grey Matter achievement (by obtaining 25 Headshots as Sniper) will not count towards any of the Sniper milestones).
Valve Corporate Achievements
|Welcome to the Internets!|