The Automap in use on the 9th level in Diablo.
The Automap is a tool used in Diablo, Diablo II and Diablo III to better navigate the game. In the first two games, it is seen as a faint overlay, accessible by hitting the "tab" key. In the third game, a mini-map is always active, but a full map can be accessed by Tab key.
Many players use the map as a navigation in order to see which areas they did not explore yet.
Diablo[]
In Diablo, the automap is shown as a simple line drawing placed over the game screen to represent walls and doors that the character has seen. Since the labyrinth is "constantly changing" (randomly generated in each new game), it is the best method to track where you are, and where you have been. You can zoom in and out of the automap by pressing "-" and "+". You can also shift the map in a direction by pressing an arrow key. Closing and reopening map will center it on the character again.
Diablo II[]
Diablo II automap
The same principle is applied here as it was in the original game, except for the map is more detailed: it now also shows Waypoints, reward chests, key quest points, types of entrances / exits, its color changes depending on layout. Arrow keys can still be used to move the map around.
Because the map is not reset until you change a game (even if you save and exit), the automap also remains as it was until the levels are re-randomized.
The automap can be displayed either as an overlay of the whole screen, or set to a "minimap", where it is set in a corner of the screen and contained in a square boundary.
Diablo III[]
Diablo III automap. Red dots are quest enemies.
In Diablo III, the map has been reworked. Instead of overlaying map, a small portion of it is shown in the upper right corner of the screen. If you wish to access a full map of the level, you can use a Tab key, gaining ability to move it around (using mouse), with 1 key recentering map at your character. The map can also be zoomed.
Full map of the level in Diablo III.
There are also maps of the Act and of the World, accessible by M key. Right click on any map will bring you one level higher (Level - Act - World). World map is only available in Adventure Mode, though.
Clicking a Waypoint will teleport you to that location as if you were using a Town Portal.
In addition to showing the terrain and entrances / exits, automap is also revealing locations of Pools of Reflection, Healing Wells, Shrines, and highlights the quest markers (showing a direction with an arrow if necessary). Allies and quest enemies, even bosses, are also shown on the map.