Leather armor is a very common item of equipment in almost any Fantasy Role-Playing Game, not to mention in other genres and in medieval Europe.
When you hear the words "leather armor" you are probably thinking about something flexible like boots or a coat. Leather armor is anything but flexible. This armor is made by dipping leather into boiling oil and stretching it over a model of a human torso. It then hardens, creating a very stiff breastplate. This is most often combined with shoulder pads and soft leather leggings.
Leather armor is by far the most common type of armor in almost all fantasy worlds. Only the most primitive cultures lack the skills needed to create this most simple type of protection. Any leatherworker or armorer can quickly produce many sets of leathers as the need arises. The primary material is a byproduct of medieval life, meaning that even the poorest militia member will assuredly own at least one set of leather armor.
This armor is very durable, being easily cleaned and resistant to the weather. Under light daily usage, leather armor can last for 6 months or more, and can be stored unworn for years without the fear of rusting like metal armors do. However in heavy combat situations, leather armor will have to be replaced as often as once a week. It is not uncommon for an army to seize all the cows and sheep in an invaded land, simply to make replacement armor.
This is the armor of choice for those of more disreputable persuasions (thieves). Leather armor is easily silenced with a few bits of extra cloth around the buckles and a shirt thrown over it for good measure. The light weight of leather allows the nimble rogue to even do moderate wall climbing and other activities, while still maintaining a level of protection from his enemies.