An anchor is a heavy object, often made out of metal, that is used to attach a ship to the bottom of a body of water at a specific point to act as a weight to keep the ship from moving. There are two primary classes of anchors—temporary and permanent. A permanent anchor is often called a mooring, and is rarely moved; it is quite possible the vessel cannot hoist it aboard but must hire a service to move or maintain it. Vessels carry one or more temporary anchors which may be of different designs and weights. A sea anchor is an unrelated device: a drogue used to control a drifting vessel.

The vessel is attached to the anchor by the rode which is made with chain, cable, or mooring line (or a combination of these). The hole in the hull through which the anchor rode passes is called "hawsepipe" because thick mooring lines are called "hawsers".

An anchor works by resisting the movement force of the vessel which is attached to it. There are two primary ways to do this—via sheer mass, and by "hooking" into the seabed. While permanent moorings can use large masses resting on this seabed, this is not practical for temporary anchors which need to be stowed onboard, so almost all temporary anchors are of the type which have metal flukes which hook on to rocks in the bottom or bury themselves in soft bottoms.

An interesting element of anchor jargon is the term aweigh, which describes the anchor when it is hanging on the rope, not resting on the bottom; this is linked to the term to weigh anchor, meaning to lift the anchor from the sea bed, allowing the ship or boat to move. An anchor is described as aweigh when it has been broken out of the bottom and is being hauled up to be stowed. Aweigh should not be confused with under way, which describes a vessel which is not moored to a dock or anchored, whether or not it is moving through the water. Thus, a vessel can be under way (or underway) with no way on (i.e., not moving).