AKA Map tack
A map pin is a small pin with a spherical head. It's different from other types of push pins in that, while it is intended to be pushed in with your thumb, it doesn't have a flattened head. It can get away with this largely because it isn't intended to hold anything up, and therefore it doesn't need to be pushed in firmly, and probably isn't being pushed into a hard backing.
There is also a form of pin, also referred to as a map pin, that does have a flat head; the head is a flat plastic frame, in the middle of which is a small piece of paper, on which one can write. These are great if you don't have the budget to buy the pre-numbered map pins (those do have the spherical head). They have a longer pin than traditional push pins, to raise them up from the map.
As you may have guessed, a map pin is used to mark positions on a map. We see them in movies all the time, especially spy and war movies. I have never seen one used in real life. I would absolutely use them myself, if only I could find a good reason.
But here's a cool usage; Mensa traditionally sent a little yellow map pin to all of its new members. This was to be worn in the lapel, a secret sign to let Mensa members recognize each other.
References:
http://mpedia.dan.info/index.php?title=Little_yellow_map_pin
Google image search.