Map markers are marked locations on the world map or town map.
In Fallout and Fallout 2, map markers are green circles on the world map, some of which can, on re-entering a given location, also be divided into sub-locations, marked with a green triangle on the town map.
In Fallout 3, map markers enable fast travel to the location 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 10 more marked locations in the Wasteland and another 52 on separate world maps, making for a grand total of 225 marked locations (224 without Lawbringer).