Hinged doors can open inward or outward, and they can be used in single door units, as French doors (with two doors sashes hinged on the side jambs and meeting in the middle) or as terrace doors (with a fixed sash on one side and a swinging sash hinged to a mullion in the center).
Sliding doors are most commonly used as entry doors leading to a patio or deck. Patio doors consist of two full glass sections, one fixed and the other sliding open and closed in upper and/or lower tracks separate from the tracks holding the fixed door.
Sliding closet doors are can hang from a track at the top of the opening, or rest on a track at the bottom of the opening. Bi-fold doors are also common for closets. They are hinged vertically in pairs, run on an upper track, and require less swing space than a hinged door.
Pocket doors run on an upper track and slide into an opening within a wall. They require no swing space.
An individual door is called a door blank. A pre-hung door is a door blank that is already hinged and hung on a fully-assembled set of side and top jambs (plus a threshold along the bottom if it's an exterior door). Door casing (trim) is often installed on one side of the jambs.
Flush doors are smooth-surfaced and generally cost less than panel doors. Panel doors have inset panel pieces that give a more elaborate, sectional look. Panel doors generally cost more than comparable flush doors.
Solid wood doors are made of all-wood construction and can be made with or without panel inserts. Less expensive are solid-core doors, which have an MDF or particleboard interior covered by wood or fiberglass veneer. Hollow-core doors have a cardboard baffle center for rigidity, covered by a veneer. They're inexpensive and are used only inside.
Steel doors and fiberglass doors are used primarily for exterior doors. An exterior door on a garage or other utility space is usually called a service door. A fire door goes between a house and an attached garage and is usually made of steel. It must have a certain fire-rating to keep a fire that starts in the garage from spreading too quickly to the interior of a home.