Armor and clothing are wearable items which increase chances of survival or add special attributes. There are many types of armor and clothing ranging from Vault jumpsuits to power armor, with power armor being the most useful.
In Fallout, Fallout 2 and Fallout Tactics, armor helps the player character by increasing their Armor Class, Damage Threshold and Damage Resistance. Different kinds of armor are better against different types of Damage.
In Fallout 3 and Fallout: Brotherhood of Steel, there is only Damage Resistance and all kinds of armor are equally resistant to all types of damage, with the exception of a few such as the Enclave Hellfire armor which increases fire resistance.
Heavier armor weighs a lot more than normal clothes, so it is important to have plenty of spare room to accommodate its weight. Heavier armor will usually slow one down where lighter armor will not affect one's agility. Heavy armor will increase noise volume and decrease chances of successful sneaking.