A Minimap is a small map, that shows the player the immediate area around her.
Minimap in Diablo 3
In the GamesCom 2009 gameplay video, the minimap can be clearly seen in the top right corner, it is much smaller than the one in Diablo 2. It is unknown if it can be edited by some options like the map in Diablo 2, which could be made transparent, opaque, smaller or bigger to a certain degree.
Minimap in Diablo II
Diablo 2 had a large transparent minimap, that covered the centre of the screen. It showed the player various important places, such as Waypoints, Shrines and caves.