Exit tiles are used to move creeps from one room to another. They are found on the edges of rooms in places where there are no natural walls. If a creep is on an exit tile and does not move, it will be transported to the corresponding exit tile in the adjacent room. This often causes problems when moving between rooms. If your creep is stuck on the edge of a room, it may not be programmed to move when it is in the other room. This will cause the creep to move back on the next tick.
Portals are a special type of tile that acts like an exit tile. They appear randomly in the center rooms of sectors, and they transport creeps that enter them to a matching portal in another sector. The portal connections change once in a while.