Regular Attacks
A melee attack is when the player uses their weapon/fist or any blunt object to get a kill. Melee attacks are a staple of the Halo series and can get even the most experienced player out of the tightest of jams.
Simple use the B button (From Halo 1 through to ODST, Right Bumper in Halo Reach)) to melee attack. Be it with your Spartan's fist, a flag or bomb, or simply the butt of his rifle. A single melee attack will result in the enemies shield being instantly depleted. Hitting them again will result in an instant kill. Using your gun or fist will warrant 2 hits for a kill. Using a flag or bomb will be an OHK.
 Jump Shots
It's important to note that jumping before you melee attack someone will result in a lot more damage. If the enemy has even half shield, this will kill them. A jump strike with a bomb or flag will still result in a one hit kill.
These are melee attacks from behind. They are always a one hit kill, no matter what you use to melee your enemy with. Holding the Melee button in Halo Reach will result in a longer and flashier animation, called an assassination. The only difference being that the player ((Spartan or Elite)) will take out a hidden knife and kill the enemy with it. Note that assassinating the enemy takes a couple of seconds, and you are not invincible while you do it, so use it wisely, because it could lead to you being killed too.