If you're on about the campaign mode...
There's legendary, which is pretty hard, and then there's skulls, each of which have nice effects such as - only recovering sheild through melee attacks, enemies love grenades, i think there's all enemies have active camo.
If you turn on all the skulls, that's said to be 'mythic' difficulty level.
f**k that! I have Grunt's Birthday Party on so I know when I've done a headshot, and IWHBJD, which isn't as cool on this cause you can't understand the enemy. They used to sing songs with it on