A Permanent Map (also called "Perm Map" or "Protected Map") is a map, that cannot be removed. Therefore, you cannot vote for it. Opposite to official maps, permanent maps occur with the same rarity as user made maps.
If you own a permanent Map and open it in the Map Editor, you aren‘t allowed to change it, there is a banner which reads „This map is now permanent, you cannot change it.“.
Ingame, you can recognize a permanent map by looking at the creator‘s name in the top left corner of the map. If the map is permanent, the name will be written in yellow and there will be a cheese in front of the text. Attention: If there is a glowing cheese, the map is official.
A list of most permanent maps can be found here. A permanent map can be gained by taking part in the Map Maker Contest for example.