In Call of Duty Modern Warfare 3, I see people with level 80. I never see anyone passed 80, so is 80 the highest rank that you can obtain?
The highest rank you can achieve in Modern Warfare 3 is 80, before prestige. When you reach level 80, you can still gain exp to continue "levelling up", but instead of reaching level 81, you are given the choice to enter Prestige Mode.
When you reach max level, you unlock something known as "Prestige Mode." You have the choice of entering prestige mode and resetting your rank back to 1 and removing all Create-A-Class unlocks and Challenges. A special Prestige Emblem will take the place of your rank emblem, promoting you up a prestige, but lowering you down all levels. You can do this up to ten times for ten prestiges.
Here is a list of all the unlocks and level requirements for levelling up in COD: MW3
Yes, Level 80 is the maximum level in multi player for this game. You can further rank up by starting prestige mode, where you level everything again for the sake of an upgraded icon.