The Mark IV armor is a heavy and complex suit that is set for ground infantry for the United Nations Space Command. The armor is designed to be worn by a Spartan 2.
Mark IV helmet is the most important part of the Mark IV. The helmet has a visor with HUD that is polarizing so that you cannot see the wearer's face. . On the side of the helmet there is a small rechargeable flood light, which is very useful in a combat situation when you're in poorly lit areas. The helmet is solar powered, and recharges from the energy from the sun or from the Mark IV Combat Suit.
Unlock Mark IV Armor:

The Close Quarters Battle Armor is the more common name for the Mark VI MJOLNIR Powered Assault Armor/ C variant armor. This armor is specialized for powered battle, to improve the survivability during close combat for a Spartan 2.
CQB Helmet features a "T" shaped visor with two large cheek guards. On the top of the helmet there is a block alloy that forces a protective brace on the top of the helmet. There is a visor on the helmet that assists the Spartan by focusing his sight to a target.
The CQB Shoulder pads are made of a rough rectangular plate going across the upper arm. It incorporates a light.
The CQB Body consists of a chest plat that is made of a silvery alloy with an additional 4 set of plates that are fused together. There is an unusable combat knife attached to the plate, too.
Unlock CQB Armor:
CQB Head - Available from the start.
CQB Shoulders - Available from the start.
CQB Body - Available from the start.

The Explosive Ordnance Disposal Armor is also known as the EOD Armor.
The EOD Helmet is a mostly squarded helmet with two separate eye pieces.
The EOD Shoulders looks a little like those of the Space Marines from Warhammer 40,000, except they are much smaller along the joints between the shoulders. These should plates are squared off and lack any trim.
The EOD Chest looks very similar to the Mark V chest piece.
Unlock EOD Armor:
EOD Head - You must complete the campaign mode on Legendary.
EOD Shoulders - You must complete the 'The Ark' level on Legendary.
EOD Body - You must complete the 'Tsavo Highway' level on Legendary.

The EVA Helmet looks a lot like the SPI Helmet, except the rim is thinner and around the back of the head has a quater- sphere shape. This helmet tends to provide better peripheral vision, yet this does not give any advantages to the player because the screen doesn't surround the player's head.
The EVA Shoulder feature a long attachment in their center.
The EVA Chest is composed of two pieces and straps betwen the back and front plate. The objects that make up the chest plate help the wearer during EVA operations.
Unlock EVA Armor:
EVA Head - You must complete the campaign mode on Normal, Heroic, or Legendary.
EVA Shoulders - You must complete the 'The Ark' level on Normal, Heroic, or Legendary.
EVA Body - You must complete the 'Tsavo Highway' level on Normal, Heroic, or Legendary.

The Scout Helmet features a cap- like protrusion along its rim, a beak- like extension under the chin, and a thin, slit-shaped visor.
The Scout Shoulders are similar to the original marines from Halo CE, except they are switched around a bit to form the shape of an 'A'.
The Scout Chest plate is similar to the EVA and the CQB chest plates because it is just an attachment. The Scout chest plate is the shape of an 'H'.
Unlock Scout Armor:
Scout Head - You must unlock the 'Used Car Salesman' achievement.
Scout Shoulders - You must unlock the 'Mongoose Mowdown' achievement.
Scout Body - You must unlock the 'Too Close to the Sun' achievement.

The Hayabusa Helmet is an upwards trio of spikes on the top and one spike on either side. The face plate of this helmet is similar to that of the Elites.
The Hayabusa Shoulders are three metal plates that are arranges in overlapping manner.
The Hayabusa Chest piece features a thick round collar that protects the neck. There is an angular strip of armor that straps down over the chest.
Unlock Hayabusa Armor:
Hayabusa Head - You must claim all 13 hidden skulls.
Hayabusa Shoulders - You must claim 9 of the hidden skulls.
Hayabusa Body - You must claim 5 of the hidden skulls.

The Mark V Head is only worn by a Spartan 2.
The Mark V head is the prototype that replaced the Mark IV helmet.
The features of the Mark V helmet is similar to that of the Mark IV.
Unlock Mark V Head:
Mark V Head - You must unlock the 'USNC Spartan' achievement.

The ODST is typically worn by ODST's.
The ODST Body Suit is light weight, yet still offering a great deal of protection. This armor can protect rounds that are 5 mm and smaller. It contains enhanced communications. Because it is very light weight, the suit aids in increased agility and manueverability.
But, on the other hand the ODST body suit is uncomfortable aver long periods of time in the field. It is ineffective in desert and arctic environments. The suit is better for nighttime, urban, forest, and space operations. Also, the ODST suit does not protect against rounds that are larger than 5mm.
Unlock ODST Head:
ODST Head - You must unlock the 'Spartan Graduate' achievement.

The Rogue Helmet is purely cosmetic. This helmet is much more rounded with a longer, thicker mouth guard than the Mark VI.
There are no shoulders or chest pieces to go with the Rogue helmet.
Unlock Rogue Armor:
Rogue Head - You must unlock the 'Spartan Officer' achievement.

The Security Helmet has an eight shaped visor. When wearing the helment will add a small antenna to the back of the armor.
The Security Shoulder are rather large with a distinct round section with an upper center.
There is not a Security Chest piece.
Unlock Security Armor:
Security Head - You must unlock 49 of the 49 achievements.
Security Shoulders - You must unlock 39 of the 49 achievements.

The Katana Body is only unlocked once you've received 1000 out of 1000 gamer points. The sword associated with the Katana resembles the Hayabusa's Dragon Sword.
Unlock Katana:
Katana - You must unlock 49 out of the 49 achievements.

The Recon Helmet has very round features with a rise along the top of the head. There are two black triangles that point out the front of the head on either side. The helmet has a thin visor and an area of black alloy that is beneath it.
The Recon Shoulders are an angular version of the EVA. They are box- like around the shoulder pads and have a section that juts out and down the arms.
The Recon Chest is similar to the Mark VI chest piece but it is made of smooth plates that resemble the human body.
Unlock Recon Armor:
Recon armor- Is available for Bungie Employees and community members who receive it as a reward from Bungie.