A quoin is an angle at the outside corner of a building. You can call the corner itself a quoin, or use the word for the special stones or bricks that reinforce corners of brick or stone buildings.
You're most likely to come across the word quoin in a discussion of architecture, since it's a term that has to do with either the aesthetics or strength of a building. Some quoins are decorative features, providing variety and pattern to the corner where two exterior walls meet. Others have an important structural job, strengthening buildings by reinforcing the corners. Quoin was originally an alternative way to spell coin, and was used to mean "cornerstone" or "wedge."