Map markers are marked locations on the world map or town map.
Fallout and Fallout 2
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.
Fallout 3
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).
List of map markers in Fallout 3
Fallout New Vegas
Map marker in Fallout: New Vegas function the same way as in Fallout 3.