A fixture is something attached to real property in a manner that makes it inalienable from the real property.
Generally, there are two types of property: real property and personal property. A fixture is something that before it was attached to real property is usually considered personal property, but through attaching it to real property, it becomes part of that real property.
A fixture is most often at issue between the buyer and seller of real property. For example, a shed might be built onto a piece of land, and the seller of the piece of land wants to transport the shed away from the land after selling it. A buyer may be able to compel the seller of the land to leave the shed if the buyer can successfully argue that the shed is a fixture, and properly considered part of the land.