You need 100 WHITE markings on your map.
Black markings are for the locations you haven't actually visited, even if you've got them on your map or compass.
A good tip for getting around quickly is using the stables. Some have carriages you can use to get around to the major Holds (for 50 gold). That puts you in a position to discover even more locations. You can buy or steal a horse to make getting around "on foot" even easier.
There are well over 100 locations available in Skyrim. There are closer to 300 than 100.
Once you discover your 100th location, the achievement pops immediately. You can check your progress at any time by pressing start and going to the Stats tab.
EDIT: Here's a link to a map from the comments: http://www.uesp.net/maps/srmap/srmap.shtml