Some gametypes involve Spartans. In ODST Infection, the Last man becomes a Spartan II with Overshields and a little faster speed. In ODST Escort, the VIP becomes a Spartan III. Although not as strong as a regular ODST for balance reasons, they wear their SPI (Semi-Powered Infiltration) Armor and have Active Camo.
The Covenant play a little differently than ODSTs. They aren't as strong in combat as ODSTs, since they only have 100% damage resistance. However, unlike ODSTs, they do have self-regenerating shields, so guerilla tactics can become lethal to ODSTs. At the start of each round, Covenant soldiers might feel a little uncomfortable, since they die right away. This is to initiate their respawn traits so that they can pick up weapons. Don't worry, you aren't losing any points. After that, they spawn in a area closed off from the rest of the map. There they have five different classes to choose from. These classes change on each map, so your always playing something new. I'll explain the classes in a moment. After choosing a class, they go into a teleporter that randomly puts them on the map. The teleporters are generally hidden, however a watchful ODST might see one appear, and attack it instantly.
Jackals are the long range infantry of the Covenant. They generally spawn with a Carbine and in some occasions, a Beam Rifle. They also have a Deployable Cover on some maps that can become temporary relief from ODST fire. Since their weapons have such longe range, one of the most popular tactics is to shoot from the tops of buildings and then back away when thy begin to get shot at
Elites are a very strong support class. They usually spawn with three Plasma Rifles, although sometimes they carry a Needler, which is surprisingly powerful against ODSTs. They also carry a choice between one of three equipment: Bubble Shield, Regenerator, or Deployable Cover and one Plasma Grenade. Many Elite classes also have a unique teleporter that carries them to powerful vehicles spawns, which can turn the tide of battle. Their Plasma Rifles are effective against ODSTs, and inside a Regenerator can become quite a threat.
Brutes usually have a variety of weapons at their disposal. They carry a Brute Shot, Spiker and Spike grenades. Some maps also use a personal teleporter that spawns way at the top of the map, so they can drop down as if they were using Jet Packs. Some also carry Flares which temporarily blind ODSTs, and others have been seen holding Radar Jammers
Elite Honor Guards carry an Energy Sword and have Active Camouflage. These soldiers are great at killing ODSTs who camp. Remember, that it takes
two sword slashes to kill with a sword, so don't expect to kill on the first hit.
Brute Chieftains carry the most weapons of any Covenant class. They have Overshields, a Gravity hammer, a Plasma Cannon, a Flame Grenade, Power Drain, and a Spiker. These beasts are definitely a threat, especially when combined with an Elite holding a Regenerator. Again, it takes
two hammer kills to kill an ODST though.

Hunters aren't very common in ODST games, but are a very strong presence on the Battle Field. They hold a single Fuel Rod Cannon and have a Set of Overshields. Killing these threats should be a top priority for ODSTs.
Brute Stalkers are the Brute version of Elite Honor Guards. They wield a single Mauler and have Active Camo. While they don't have a lung like Honor Guards do, their Maulers do have quite a range.

The last Covenant class is the Arbiter. The Arbiter has a set of Overshields, an Energy Sword, and a Plasma Rifle. The Arbiter is only seen on one map, but is a big threat if used properly.
Health Packs

Health Packs are used to replenish health for ODSTs. In the form of Custom powerups, they play a huge role in the game. The settings for Health Packs are 100 % shields regeneration for 10 seconds. The picture above shows an ODST picking up a Health Pack at the right time: when he is 'one shot'. Health Packs respawn in a different method than most equipment and weapons in the game. They have a 3 minute respawn timer and aren't placed at the start of the game. However, the run time minimum is set at one, so there will always be at least
one Health pack in the city at a time. Picking it up will automatically activate another Health pack somewhere else in the city. 25% of the time it might just come back to where you picked it up. If a Health pack location doesn't respawn for 3 minutes, it is force-spawned and then there may be two health packs in the city at a time.
Since Covenant soldiers already have this trait, picking up a Health Pack has no effect on them whatsoever. Guarding the Health Packs would be a better strategy than picking it up like this Honor Guard shown above. Another strategy that occasionally works is standing on each location of each Health Pack so ODSTs can't pick up any. This is hard to pull off, but can make it a lot harder for ODSTs
The Gametypes
Now you know how the classes work in ODST, I'm going to go a bit more in depth about the gametypes: ODST Infection V2, ODST Escort V3, and ODST Slayer V2
Shadows on the walls
In ODST Infection V2, a quarter of the players start as Covenant soldiers. As mentioned above, they die at the beginning of the round. Covenant soldiers have a
slight damage decrease since the ODSTs will soon be facing a whole horde of Covenant soldiers. Since this is considered a Free-For-All gametype, teamwork isn't
as essential compared to the other gametypes, but it still is important. Remember, ODSTs do not have the ability to drive vehicles. Once there is only one ODST left standing, he becomes a Spartan II. The Spartan has Overshields and still has 200% damage resistance that the ODSTs had. He is also able to run a bit faster than the Covenant soldiers, so use this to your advantage. There a three rounds of Infection.
ODST Escort is my favorite gametype of the three. Two even teams play four three-minute rounds. One player on the ODST team becomes the VIP. The VIP is a Spartan III is SPI Armor (Semi-Powered Infiltration Armor). They aren't as strong as ODSTs (for balance reasons) but they do have Active Camo, though they still have a marker over their head. The ODSTs must escort the Spartan III to the different locations throughout each city. You get two points for arriving at each destination, and the Covenant receive one point for killing the VIP.
In ODST Slayer, two teams play two rounds so each side plays each faction once. Although the gametype uses the VIP variant, it plays the same as Team Slayer; one point per kill. This means that there is not difference for the VIP, no extra points or upgrades. 15 points ends a round.