I'll start from the first rank and go up..

Rank What you unlock :O

Private Second Class - Private Package

Private First Class - Basic Training Achievement(5g), Headgear 1 (Balaclava, Tactical helmet)

Specialist - Rainbow Weapons (MP7A1, 552 Commando, USP40)

Corporal - Camouflage 1 (Desert, Urban, Russian, Guerrilla, Fall, Desert2)

Sergeant - Tactical Armor (Raven Recon, Vulture Combat, Falcon Assault)

Staff Sergeant - Headgear 2 (Three hole balaclava, baseball cap, breathing mask)

Sergeant First Class - Camouflage 2 (Flecktarn, Orange, Swedish, War2k5, Alpen, White)

Master Sergeant - Freedom Fighter Weapons (Ak47 Raging Bull, SV-98)

First Sergeant - Hedgear 3 (Boonie hat, beret, tinted goggles)

Sergeant Major - Freedom Fighter Armor (Tyhpoon, Cyclone, Hurricane)

Second Lieutenant - Officer(20g) Camouflage 3 (Pink Blue Woodland Wasp Sand Crimson)

First Lieutenant - Hedgear 4 (Half face mask, reinforced helmet, tactical goggles)

Captain - Mercenary Weapons (MG-36, SPAS-12, Desert Eagle)

Major - Military Armor (Anaconda, Viper, Diamondback)

Lt. Colonel - Camouflage 4 (Yellow Urban, Red Urban, Tiger, Rust, Urban II, Grey)

Colonel - Headgear 5 (Ballistic Face Mask, Riot Helmet, Night Vision Goggles)

Elite - Best of the Best(40g), Camouflage 5 (Custom Camouflage)

-------------------------------------------- ----- Getting to elite..

1. Realistic difficulty terrorist hunts of LVU campus is some of the fastest XP, 4 players are needed. 1.b. Shields with a high cap mag on an MK.23 or a 92FS help a lot.

2. If you can get a friend to host a dedicated match (Team survival on LVU Campus for 20 minutes with time between rounds set to 15 seconds with 2 slots only [this way no one else can get in and interfere and kill you]) and another friend on the opposite team of you. So you and your friend on the other team can go leave the game and sleep, or just do something else getting 799 experience every 20 minutes. Please note that the dedicated host will NOT get any XP.

Hope this helps!