A firearm is a portable gun that can fire one or more projectile from its usually metal barrel, often discharged by gunpowder. Firearms are also referred to as small arms when they intended primarily for military use.
A handgun is the smallest type of firearm and are rather accurate. Nearly all handguns can be used with only one hand and, therefore, can easily be dual wielded. Most handguns are semi-automatic and have low recoil, but some can be fully automatic.
A submachine gun is a small to medium sized weapon. It is always fully-automatic, and fires the cartridges of pistols. Because of this, it isn't highly accurate, especially in the case of higher distances (50m or more). Some submachine guns can be dual wielded.
An assault rifle is a versatile firearm, being able to have multiple fire modes and support for multiple attachments such as scopes, grips or even grenade launchers. Most assault rifles are fully automatic, but can be semi-automatic or burst fire. Some are able to freely switch between multiple modes of fire. Because assault rifles are heavier than sub-machine guns, the recoil caused by automatic fire isn't as severe, making assault rifles acurate and effective over all but long range combat.
There are a wide variety of different shotguns: most are pump action, but can be semi-automatic or even fully automatic and some can have lever action. Most shotguns fire 12-gauge shells which launch multiple projectiles in a spread, making them deadly in short range but nearly ineffective at medium to long range. Shotguns can be effective at longer ranges if loaded with "slug rounds"; shells that are loaded with one large projectile, making for a more accurate, longer range shot.
A sniper rifle is a highly accurate, long range rifle. It uses high calibre rounds for maximum stopping power. Sniper rifles can be bolt action or semi-automatic and are commonly seen with a scope, but can be used with other sights. Sniper rifles are heavier than most firearms and are difficult to use in short to medium range, but excell in long range combat.