In some roguelikes (and occasionally other games) the player can traverse the world using the Overworld, a graphical representation of the world in which the game is being played. By walking across the overworld, players can access various areas, allowing for a lot of gameplay variety.
Overworlds allow for more realistic travel than simply selecting a desired location.
Overworlds take two forms; realistic and symbolic. Realistic Overworlds, like the one from ToME require the player to travel to different locations using the same world scale as is found in dungeons; you walk down the roads, through the forests and so on, putting your character through the real hardships of long-distance travel. While this is more realistic, it can also be highly frustrating.
Symbolic Overworlds are, in short, an atlas that you can travel across. One tile can represent an entire town, dungeon or forest, and pressing a defined key will let you enter the area in which your character is currently present on the Overworld. This approach is used e.g. by ADOM and Omega.
Do not confuse Overworlds with Surface Worlds. Overworlds are generally much larger in scale.
For example, the starting town in Angband is a form of Surface World, as it is on the surface of the dungeon. Much like, for example, the surface town in Larn. However, the Surface World does not allow access to other towns, zones or dungeons. This distinguishes it from an Overworld, the primary function of which being transit between locales.