I got the Explorer perk just now, at Level 20 and my mind = blown when I looked at the map.
@Tysy: The compass has small bars that represent NPCs, and if they're red, then they're hostile NPCs. If you see a filled arrow, that shows the direction of where you need to go next for the current quest you're following (click on a quest in the Quests section of Pipboy). If the arrow isn't filled, then it's a marker set by you. You can set markers on the World Map by clicking at any location that you haven't discovered yet. If you have, then you can teleport there instead.