A gallery is an area of a building that's usually long, narrow, and has a specific function. You might visit an art gallery to check out a row of paintings hung on its walls.
There are a few kinds of galleries, but the first is a part of a house or building that's unusually long and narrow. A gallery also has some sort of purpose: in a theater, a gallery is a place where additional seats are located. A gallery is also somewhere art is shown and sold. Art galleries can be one room of a larger place, a series of rooms, or a whole building devoted to art.