Q: What is a chicken coop?
A: The word "coop" simply refers to the structure that your chickens live in. A run refers to the enclosed part of the chickens' area that is outside, ideally with access to pasture.
In hot dry areas, sometimes coops are three-sided, with the fourth wall made of welded wire for security against predators. The "missing" wall provides extra ventilation and makes sure the coop doesn't get too hot inside. In cold areas, they can be quite small. A coop can be a purpose-built structure or a converted shed or building, made of wood, plastic, adobe, etc. They can be large or small.
Small coop with attached run.
Large coop and run, attached to house
A coop can be made from brand new materials or recycled ones!
Coop built out of recycled pallets
Coops can even be mobile. A mobile coop meant to be moved every day to new pasture is often called a "chicken tractor."
Mobile chicken coop, or chicken tractor
The coop may have a small attached "run," meant to give your flock some outdoor access at times when you can't be there to let them run in a larger area...
Adorable small coop with small run
A run could also just be a large fenced area that surrounds the coop. Sometimes you might even have a mobile, enclosed run. This allows you to have a large, stationary coop, but to still move your birds to fresh pasture every day.
Movable chicken run