Below you will see all the medals available in Halo 2 online.
Click on the screen dumps as evidence that I have been awarded these medals.

These screen shots show me as YardDealer, which was my original gamertag.

This table was last updated on 03/06/05
 

Killing Spree
Award: Kill 5 opponents in a row without dying
Availability: All gametypes
Visual Representation: a steel 4 sided star
Difficulty Factor: 6
     
Running Riot
Award: Kill 10 opponents in a row without dying
Availability: All gametypes
Visual Representation: a bronze 5 sided star
Difficulty Factor: 7
     
Rampage
Award: Kill 15 opponents in a row without dying
Availability: All gametypes
Visual Representation: silver 6 sided star
Difficulty Factor: 8
     
Berserker
Award: Kill 20 opponents in a row without dying
Availability: All gametypes
Visual Representation: a gold 8 sided star
Difficulty Factor: 9
     
Double Kill
Award: Kill 2 opponents within 4 seconds of each other
Availability: All gametypes
Visual Representation: brass medal with 2 stars
Difficulty Factor: 5
     
Triple Kill
Award: Kill 3 opponents within 4 seconds of each other
Availability: All gametypes
Visual Representation: steel medal with 3 stars
Difficulty Factor: 6
     
Killtacular
Award: Kill 4 opponents within 4 seconds of each other
Availability: All gametypes
Visual Representation: bronze medal with 4 stars
Difficulty Factor: 7
     
Kill Frenzy
Award: Kill 5 opponents within 4 seconds of each other
Availability: All gametypes
Visual Representation: silver medal with 5 stars
Difficulty Factor: 8		  
     
Killtrocity
Award: Kill 6 opponents within 4 seconds of each other
Availability: All gametypes
Visual Representation: gold medal with 6 stars
Difficulty Factor: 9		  
     
Sniper Kill
Award: Kill an opponent with the sniper rifle or beam rifle
Availability: Games with sniping weapons
Visual Representation: 6 pointed star with crosshairs
Difficulty Factor: 1
Message: You sniped (player)
     
Roadkill
Award: Hit and kill an opponent with a vehicle
Availability: Games with vehicles
Visual Representation: 5 pointed star with a wheel
Difficulty Factor: 1
Message: You splattered (player)
     
Bonecracker
Award: Hit and kill an opponent with a melee attack
Availability: All games
Visual Representation: Inverted star with a fist
Difficulty Factor: 3
Message: You beat down (player)
     
Assassin
Award: Melee an opponent from behind
Availability: All games
Visual Representation: Ninja on a throwing star
Difficulty Factor: 2
Message: You assassinated (player)
     
Stick It
Award: Kill an opponent by sticking him with a plasma grenade
Availability: Games with plasma grenades
Visual Representation: Active grenade on a 16 pointed star
Difficulty Factor: 3
Message: You stuck (player)
     
Carjacking
Award: Steal an occupied vehicle from an opponent
Availability: All games with vehicles
Visual Representation: Cross with a U-Turn
Difficulty Factor: 2
Message: You boarded an enemy vehicle
     
Flag Carrier Kill
Award: Kill an opponent carrying a flag
Availability: CTF Games
Visual Representation: Hexagon with targeted flag
Difficulty Factor: 3
     
Bomb Carrier Kill
Award: Kill an opponent carrying the bomb
Availability: Assault games
Visual Representation: Hexagon with bomb
Difficulty Factor: 3
     
Flag Taken
Award: Take an opponent's flag from their base
Availability: CTF games
Visual Representation: Yellow triangle with flag
Difficulty Factor: 4
Note: Only seen in post game
     
Flag Returned
Award: Return your flag to your base
Availability: CTF games with touch return enabled
Visual Representation: Green circular medal with flag
Difficulty Factor: 1
Note: Only seen in postgame
     
Bomb Planted
Award: Plant a bomb in opponent's base
Availability: Assault games
Visual Representation: Red inverted triangle with bomb
Difficulty Factor: 6