Map markers are marked locations on the world map.
Fallout 3
In Fallout 3, map markers can be fast travelled to once discovered.
Without add-ons, there are 163 marked locations in Fallout 3 provided you have the Lawbringer perk; without it there are only 162 as the Regulator HQ is not available. With Operation: Anchorage, The Pitt and Broken Steel there are 8 more marked locations in the Wasteland and another 21 on seperate world maps, making for a grand total of 192 marked locations (191 without Lawbringer).
List of map markers in Fallout 3
See: Fallout 3 places
See: Fallout 3 map