Call of Duty can be a very hectic multiplayer First Person Shooter.
When it comes to identifying friendly players from enemy players, a name highlights over their head when you are looking at them. If they are on the enemy team, this name will be red, and if they are on your team, this name will be green.
On top of this, there are UAV's (Unmanned Arial Vehicles) which players may call in when they have managed to kill 3 enemies without dying. These will show a little minimap which will display everyone's location on either hardcore or normal servers. (hardcore servers remove the Heads Up Display and any information like ammo reserves)
Here, you can see members of the OpFor team:
They are always wearing dark coloured clothing, and wear hats or headdresses that are red.
In the first screenshot, you can also see that the UAV map is active, (top left corner) indicating where your teammates are (green arrows) and where your enemies are (red dots).
Here, you can see members of the American team:
Their clothing is light in colour; what may be considered 'desert cammo'. They wear helmets.
You can once again see that the UAV is active, and showing where your friendly soldiers are VS the enemy soldiers. It also displays where the helicopter is located, and whether or not it is friendly. (friendlies are always green, enemies are always red on the UAV, including helicopters or airstrikes)
Call of Duty 4 consists of a few game modes.
- Free-for-all means that it's every man for himself; you shoot people no matter what clothing they are wearing.
- Team Deathmatch means that you simply shoot people on the opposite team, and try to beat the enemy team's score.
- Capture the Flag involves grabbing a flag from the enemy base, and bringing it all the way back to your base.
- Headquarters involves capturing a location on the map and holding it for as long as possible. This location changes, so you need to capture as many as you can.
- Domination involves capturing and holding as many locations on the map as possible. There are always 3 persistent locations your team can capture.
- Search and Destroy has no respawns. You are either on the defending team, or the attacking team. Attackers try to plant a bomb and set it off at a bomb site, defenders need to prevent the attackers from blowing up the bomb site.
- Sabotage is similar to Search and Destroy, except there are respawns, and both sides have an objective to blow up, and an objective to defend. There is also one single bomb which can be dropped and picked up by the enemy team.
If you are new, I suggest avoiding 'Hardcore' servers, as you do not get a minimap, and your health is 40% of what it is in a normal match, so you will die much faster if you aren't quick on the trigger.
If everything is too fast for you though, a hardcore server can have it's advantages. People generally pace themselves slower on hardcore because they can die much faster if they run around without checking corners and being careful. It means that it shouldn't be as hectic, but you will die easier.
For some good general tips, you might try looking here:
How can I become a better CoD player?