A theater, theatre or playhouse, is a structure where theatrical works, performing arts, and musical concerts are presented. The theater building serves to define the performance and audience spaces. The facility usually is organized to provide support areas for performers, the technical crew and the audience members, as well as the stage where the performance takes place.

There are as many types of theaters as there are types of performance. Theaters may be built specifically for certain types of productions, they may serve for more general performance needs or they may be adapted or converted for use as a theater. They may range from open-air amphitheaters to ornate, cathedral-like structures to simple, undecorated rooms or black box theaters. A thrust stage as well as an arena stage are just a few more examples of the multitude of stages where plays can occur. A theatre used for opera performances is called an opera house. A theater is not required for performance (as in environmental theater or street theater), this article is about structures used specifically for performance. Some theaters may have a fixed acting area (in most theaters this is known as the stage), while some theaters, such as black box theaters have movable seating allowing the production to create a performance area suitable for the production.

A theater building or structure contains spaces for an event or performance to take place, usually called the stage, and also spaces for the audience, theater staff, performers and crew before and after the event.

There are usually two main entrances of a theater building. One is at the front, used by the audience, and leads into a foyer and ticketing. The second is called the stage door, and it is accessible from backstage. This is where the cast and crew enter and exit the theater, and there is a tradition called "stage dooring" that some fans participate in, in which fans wait outside of the stage door after the show in hopes of getting an autograph from the actors.

The acting or performance space is the stage. In some theaters, such as proscenium theaters, arena theaters and amphitheaters, this area is permanent part of the structure. In some theaters the stage area can be changed and adapted specifically to a production, often called a black box theater, due to the common practice of the walls being painted black and hung with black drapes.

Usually in a building used specifically for performance there are offstage spaces used by the performers and crew. This is where props, sets, and scenery are stored, and the performers standby before their entrance. These offstage spaces are called wings on either side of a proscenium stage. A prompter's box may be found backstage. In an amphitheater, an area behind the stage may be designated for such uses while a blackbox theater may have spaces outside of the actual theater designated for such uses.

Often a theater will incorporate other spaces intended for the performers and other personnel. A booth facing the stage may be incorporated into the house where lighting and sound personnel may view the show and run their respective instruments. Other rooms in the building may be used for dressing rooms, rehearsal rooms, spaces for constructing sets, props and costumes, as well as storage.

All theaters provide a space for an audience. In a fixed seating theatre the audience is often separated from the performers by the proscenium arch. In proscenium theaters and amphitheaters, the proscenium arch, like the stage, is a permanent feature of the structure. This area is known as the auditorium or the house.