The Emblems are a way of ranking your accomplishments upon beating the game. It’s pretty similar to the ranking used in MGS 3. You can earn several Emblems upon meeting several cateria during your play though of the game. Typically, one Emblem appears first and is considered by players to be their "main" for that run through.

Others will also appear for your various play styles and accomplishments. There are 40 in total, and collecting all of them not only gives you plenty to do with MGS4, but also rewards you with some other iPod tunes. The hardest Emblems to achieve are the Big Boss Emblems. Only when you achieve the Big Boss Emblems then you can be considered as one of the best Metal Gear Solid players in the world.

Emblem: Ant
How to Unlock: Hold up and frisk enemies for items 50 times

Emblem: Assassin
How to Unlock: Have 50 or more knife kills, 50 or more CQC holds, and less than 25 alerts when completing the game.
Notes: "Knife kills" are the slit throat action when holding an enemy, so CQC holds go hand in hand with knife kills, you can't do a knife kill without a CQC hold.

Emblem: Bear
How to Unlock: Grab at least 100 enemies in CQC.

Emblem: Bee
How to Unlock: Use the Syringe on at least 50 enemies.
Notes: Scanning Plug S can be used too apparently.

Emblem: Big Boss
How to Unlock: Beat the game on The Boss Extreme with no kills, no deaths, no alerts, no healing items used, and without using Bandanna or Stealth in under 5 hours.
Notes: Completing the requirements for this may award you with several other Emblems if you didn't collect them, including Fox, Fox Hound, Hound, Mantis and Raven.

Emblem: Blue Bird
How to Unlock: Hand over 50 items to rebels.

Emblem: Centipede
How to Unlock: Have less than 75 alerts, less than 250 kills, and more than 25
continues.

Emblem: Chicken
How to Unlock: Have over 150 alerts, over 500 kills, over 50 continues, use more than 50 healing items, and beat the game in over 35 hours.

Emblem: Cow
How to Unlock: Have over 100 alerts.

Emblem: Crocodile
How to Unlock: Have over 400 kills.

Emblem: Eagle
How to Unlock: Have 150 or more headshots.

Emblem: Fox
How to Unlock: Beat the game on Solid Normal, with no kills, no deaths, 5 or less alerts, no healing items used, no Bandanna or Stealth used in under 6 hours.

Emblem: Fox Hound
How to Unlock: Beat the game on Big Boss Hard or higher with no kills, no deaths, 3 or less alerts, no healing items used, no Bandanna or Stealth used in under 5 and 1/2 hours.

Emblem: Frog Emblem
How to Unlock: Roll forward at least 100 times.

Emblem: Gecko
How to Unlock: Lay against walls for a total time of at least 1 hour.

Emblem: Giant Panda
How to Unlock: Beat the game in over 30 hours.

Emblem: Gibbon
How to Unlock: Hold up 50 enemies.

Emblem: Hawk
How to Unlock: Be praised 25 times by rebels
Notes: Eliminate enemies so the rebels can clearly see, and say "Nice shot!" among other things

Emblem: Hog
How to Unlock: Experience 10 Combat Highs.
Notes: To get a Combat High, go into alert and fire any automatic gun with a lot of ammo.

Emblem: Hound
How to Unlock: Beat the game on Big Boss Hard or higher with no kills, 3 or less alerts, no deaths, no healing items used, not using Bandanna or Stealth in under 6 and 1/2 hours.

Emblem: Hyena
How to Unlock: Pick up 400 or more dropped weapons.

Emblem: Inchworm
How to Unlock: Crawl on the ground for a total time of at least 1 hour.

Emblem: Jaguar
How to Unlock: Have less than 250 kills, over 75 alerts, and less than 25 continues.

Emblem: Leopard
How to Unlock: Have less than 250 kills, over 75 alerts and more than 25 continues.

Emblem: Little Gray
How to Unlock: Obtain all weapons.

Emblem: Lobster
How to Unlock: Crouch for a total time of at least 2 and 1/2 hours.

Emblem: Mantis
How to Unlock: Have no alerts, no deaths, and no healing items used and beat the game in under 5 hours.

Emblem: Octopus
How to Unlock: Beat the game with no alerts.

Emblem: Panther
How to Unlock: Have over 75 alerts, over 250 kills, and less than 25 continues.

Emblem: Pig
How to Unlock: Consume more than 50 healing items.

Emblem: Pigeon
How to Unlock: Beat the game with no kills.
Notes: Unmanned enemies do not count.

Emblem: Puma
How to Unlock: Have over 75 alerts, over 250 kills, and more than 25 continues.

Emblem: Rabbit
How to Unlock: Flip through 100 Playboy pages.

Emblem: Raven
How to Unlock: Beat the game in under 5 hours.

Emblem: Scarab
How to Unlock: Roll on the ground over 100 times
Notes: Must have weapon equipped to do this, hold L1 and roll, then stop. Do this 100 times, making sure to stop or else it will not register new rolls.

Emblem: Scorpion
How to Unlock: Have less than 250 kills, less than 75 alerts, and under 25 continues.

Emblem: Spider
How to Unlock: Have over 250 kills, less than 75 alerts, and over 25 continues.

Emblem: Tarantula
How to Unlock: Have over 250 kills, less than 75 alerts, and under 25 continues.

Emblem: Tortoise
How to Unlock: Spend a total time of at least 1 hour in a Drum Can or Cardboard Box

Emblem: Wolf
How to Unlock: Beat the game with no continues and no healing items used.