Orbital Drop Shock Trooper

An Orbital Drop Shock Trooper, more commonly known as an ODST, is a highly-skilled and highly-trained soldier. ODSTs are members of the 105th Division of the United Nations Space Command Marines. They are a major story element of the Halo universe.

ODST (Orbital Drop Shock Trooper)
The Orbital Drop Shock Troopers, or "ODST", are an elite special operations unit within the UNSC Marines. Their role on the battlefield is usually described as that of an aggressive shock infantry raid force. As their name implies, ODSTs are dropped from low-orbital positions onto the surface of planets (or in the case of both Halo rings, ancient artifacts), often into combative situations. They are deployed via SOEIVs (Single Occupant Exo-atmospheric Insertion Vehicle, also called HEVs, for 'Human Entry Vehicle'), one-man pods designed to carry soldiers to their drop point. This unique delivery method makes them collectively less susceptible to enemy ground-to-air defenses than conventional Marines, who have to be ferried to battle in large troop carrier dropships. Once at their destination, the ODSTs have to move quickly in order to regroup, secure a perimeter, and treat the less fortunate of their ranks.

The ODSTs were first introduced and described in the novels based on the Halo: Combat Evolved storyline. However, ODSTs did not make an actual in-game appearance until Halo 2. Although they have sometimes been described as a futuristic science fiction equivalent of WWII Paratroopers, the ODST's primary role is as shock infantry assault in deep enemy territory. Probably the most well known ODST unit is NSWG2-ODST team 5, nicknamed "Helljumpers" (partly because "NSWG2-ODST team 5" is too hard to use in conversation).

The ODSTs wear a special uniform to distinguish them from other Marines. Instead of the standard green, they are clad in gray and urban camouflage, with a helmet that includes a face shield (reminiscent of the Spartans' MJOLNIR Mark VI armor).Most ODST's carry a standard combat knife in a sheath located on the back of the waistline although this is never seen to be used in gameplay. Their chest armor includes pouches and they have additional leg armor compared to the standard Marine armor in the game.This makes them instantly recognizable as elite troops.

Helljumpers
The 105th Division of the UNSC Marine Corps is an entirely volunteer outfit, and with good reason. The HEVs heat up immensely due to atmospheric friction following deployment, sometimes fatally so. Even if the ODST survives the heat, there's always the chance that his HEV won't. As experienced by Major Antonio Silva in The Flood, the pod's outer layers sometimes get stripped away, leaving the ODST to face the elements with nothing but his combat armor. The third and by far worst fate for a falling ODST is chute failure. The HEVs achieve such high velocities in descent that the ever-present threat of a parachute failure would almost certainly end in a fatal crash. The HEVs are armored, but not enough to protect an ODST from an over 400 km/h impact.
 * Note the name is reminiscent of modern day Smoke Jumpers

ODST Bravado
Also due to the huge risk of their job and the superiority of their training, the ODST's are proud and have a very "gung-ho" attitude, very similar to real-life U.S. Marines.

The ODST's are also known for being extremely brutal, even to fellow Marines. The reasons may be many, but the general theory is that they believe that other Marines are inferior for not signing up for their exclusive elite unit. It is also possible that this brutality stems from the physiological strain of repeated orbital drops - violent acceleration and intense heat are known to cause brain damage - but this is generally considered unlikely. Many also despise the SPARTAN project, calling the subjects "freaks," because many seem to believe that the bio-mechanically altered/enhanced SPARTANs are not truly human. Some ODST's view the SPARTAN's augmentations as an unfair advantage over themselves, seeing as the ODST's earned their status by their own physical prowess without any artificial enhancements. They do, however, respect the individuals as excellent soldiers. However cruel they are to the other forces, they are the cream of the UNSC crop.

The ODSTs' attitude and bravado were introduced and developed in the novels, but does not appear in Halo 2, in fact the ODSTs don't even have their own dialogue, rather the standard Marine combat dialogue is given to them, and the ODSTs in the game (for their very limited appearance) have no problem fighting alongside the Master Chief. However, it could be reasoned that they would stifle their hatred during battle.

ODST Origins
The original idea for ODST's comes from the Mobile Infantry in Robert Heinlein's Starship Troopers and from the modern day U.S. Navy SEALs -- both are highly mobile elite units used to disrupt enemy forces in support of the main assault force.
 * Their quick-deployed shock-infantry role has more in common with elite parachute-capable infantry like Army Rangers, Army Airborne units such as the 101st and 82nd Airborne Divisions, and British Commonwealth Airborne regiments