The basic weapon types are:
Assault Rifle - Fully automatic, 3 round burst or semi, often with a grenade launcher attached. Very good for quick movement and supressing. The grenade launcher adds an element of indirect fire support. Indirect fire allows you to target someone that is not at a line of sight position to you. If you can see them and take aim, they can see you and take aim on you.
Machine Gun - A machine gun allows you to lay down a heavy volume of continuous fire. With an assault rifle, a clip is changed every 20-40 rounds, with a machine gun, the belt size is usually 200 rounds. This means a gunner can put out 200 rounds continuous, in about 20 seconds (10 rounds per second) whereas an assault rifle puts out 30 rounds in 3 seconds, reloads for 2 seconds, then another 30 rounds. After the same 20 seconds, the assault rifle puts out 120 rounds. This is an effective force multiplier. 1 gunner firing constantly is almost equal to 2 assault rifles firing constantly.
Close Quarter Weapons - Different games put the CQB weapons in different categories. The shotgun and sub-machine gun are the two primary weapons for a CQB. The shotgun is effective to 100 yards, with devestating results at 25-50 yards, and very little effectiveness past 100 yards. An automatic shotgun, firing 9 00 Buck per shot, at 6 shots per second, will put out 54 .33 calibers balls a second. At 100 yards, the shotgun is putting out comparable lead to a 7.62mm mini-gun 6 barrel gatling gun. The mini-guns are capable of 50-100 rounds per second. The sub-machine guns are the MP-5 and similiar weapons. 9MM or .45 caliber automatic weapons. Their effectivve range is 200-300 yards. The most effectve part of the sub-machine guns are their silenced operation, low recoil and ease of aiming. The MP-5 is the best weapon of choice for headshots and stealth. A silenced weapon is quiet enough not to attract attention, and with good camoflouge, you can make multiple hits from the same location before being spotted.
Sniper Rifle - The sniper rifles vary greatly, from semi-auto rifles, bolt action rifles, long range 50 caliber rifles, and even an assault rifle with a low power scope. Choosing the right sniper rifle depends on the game, distance and situation. Close support sniper weapons are usually the semi-auto rifles, or assault rifles with scopes. An M-4 with a 4 power scope is very effective in the 300-400 yard range. A semi auto with an 8 power scope works from 300-800 yards. The bolt action rifles are good for 600-1000 yards. Remember, as distance increases, the bullet drop increases greatly. The difference between a 400 and 450 yard shot is minimal, the difference between a 900 and 925 yard shot is enough to cause a miss. Make sure to adjust for 10 yard increments past 700 yards or so.
Pistols - When you're down to using a pistol, you're usually in trouble. A pistol is a great backup gun for a sniper on the move, it provides more firepower than a sniper rifle, and most encounters are close range.
Explosives - This cover a wide range of weapons, from hand grenades, satchell charges, RPGs, claymores and mines. Using each one correctly can greatly increase your effectiveness in a FPS.
The individual pages will describe in more details the strategies and techniques for each weapon.