We had a list of the Achievement's icons for Halo: Reach, but now, Bungie has detailed how you get them (sans GamerScore).
There are 23 achievements related to the Campaign, 6 under "Any," 4 Multiplayer, 7 for Firefight and 9 for "Player Experience" for a total of 49 Achievements.
The "Any" achievements seemed to be related to specific actions you need to do, be it in the Campaign, Multiplayer or Firefight, hence the grouping name. The Multiplayer specific ones are few, but damn, does one of them sound difficult: "Won a game of Invasion in the 1st phase." Plus, the oft-rumored "Legendary by Yourself" Achievement is confirmed at #4 in the list, with the crossed swords and human skull.
It pretty intense to see the Achievements for the game when it's not due out for anther month and a half - plus the first TV spot for the game was just aired on my TV...creepy - and it's gives those some insight on the story.
Catch the rest after the break, but be warned: SPOILERS exist beyond the break.
- Completed the Campaign on Normal difficulty.
- Completed the Campaign on Heroic difficulty.
- Completed the Campaign on Legendary difficulty.
- Completed every mission in Halo: Reach alone, on Legendary.
- Completed the 2nd mission on Normal or harder.
- Completed the 3rd mission on Normal or harder.
- Completed the 4th mission on Normal or harder.
- Completed the 5th mission on Normal or harder.
- Completed the 6th mission on Normal or harder.
- Completed the 7th mission on Normal or harder.
- Completed the 8th mission on Normal or harder.
- Completed the 9th mission on Normal or harder.
- Completed the 10th mission on Normal or harder.
- Cleared the 2nd mission without setting foot in a drivable vehicle.
- Killed 2 vehicles at once with the Target Locator in the 3rd mission.
- Hijacked a Banshee during the Reach Campaign.
- Killed the Elite Zealot before he escaped during the 5th mission.
- Performed an Assassination against an Elite to survive a fall that would've been fatal.
- Destroyed the Corvette's engines & escort in under 3 minutes in the 6th mission on Heroic or harder.
- Finished the 9th mission on Legendary with the Scorpion intact.
- Earned a Triple Kill while Jetpacking in Campaign, Firefight or Matchmaking.
- Killed 7 Moa during the 2nd mission of the Campaign.
- Killed 3 of the anti-aircraft batteries during the 8th mission.
- Used a Health Pack to replenish life after taking body damage.
- Performed an Assassination on an enemy.
- Killed 10 enemies in a Firefight or Campaign session with the M6G pistol.
- Killed 10 enemies in a Firefight or Campaign session with the DMR.
- Traded weapons with an AI ally in Campaign.
- Killed 10 enemies in Firefight or Campaign with a supercombine explosion.
- Let a teammate spawn on you 5 times in an Invasion Matchmaking game.
- Earned a First Strike Medal in a Matchmaking game.
- Won a game of Invasion in the 1st phase.
- Earned a Killing Spree in multiplayer Matchmaking.
- Earned a Killionaire medal in Firefight.
- As an Elite, killed 5 Spartan players in Matchmaking.
- Scored 15,000 points in Score Attack Firefight Matchmaking.
- Scored 50,000 points in a Firefight game.
- Scored 200,000 points in a Firefight game.
- Scored 1,000,000 points in a Firefight game.
- Completed a Firefight set on Legendary without dying.
- Purchased an item from the Armory that required the rank of Lt. Colonel.
- Reached the rank of Corporal in the UNSC.
- Reached the rank of Captain in the UNSC.
- Used the File Browser to upload a file to your File Share.
- Completed all of the Daily Challenges in a given day.
- Purchased an item from the Armory.
- Recommended a file to someone.
- Advanced a Commendation to a Silver state.
- Completed a Weekly Challenge.