Perk 1 slot perks are Recon, Sleight of Hand, Blind Eye, Extreme Conditioning and Scavenger.
Perk 2 slot perks are Quickdraw, Blast shield, Hardline, Assassin and Overkill.
Perk 3 slot perks are Stalker, Steady Aim, Sitrep and Dead Silence.
COD MW3 perks have 3 “slots,” if you will. Each slot allows one perk to be selected. For example you can use Recon, Quickdraw and Stalker (a slot 1, 2 and 3 perk) but not Recon, Sleight of Hand and Scavenger (all slot 1 perks). Perks give you certain abilities on the battlefield that you can tailor to your needs using your custom classes (customized loadouts).
As you progress, perks are unlocked and as you use the perks, they can level up to their respective Pro levels. So there is Recon Pro, Sleight of Hand Pro, Quickdraw Pro and Dead Silence Pro. The Pro perk levels add new abilities to their counterparts.