Patterns of identical shapes that have no gaps. They usually represent the playfield in various games. They are also used in web development for reducing the count of images which needs to be transmitted.
Tiles are a very common element in games, used to store information in a simple tessellated format. Because tile grids can easily represent the space of the entire screen, tiles are a very common means of organizing level structure.
The utility of tiles ranges from the layout of ground to hard restrictions on the movement of in-game entities. It may apply to only parts of the game such as over world movement, or it could represent the entirety of all meaningful interactions in the game.
In web development they are commonly used to reduce the amount of data which needs to be transfer. They are also used for implementing a fast hover effect, where just the view port is moved to display another state.