Swarm cells are the first signs of a colony preparing to swarm.
Swarm cells are more likely to be seen in a colony in spring and summer than either emergency or supersedure cells. They are the first sign most beekeepers will see that a colony is preparing to swarm.
Swarm and supersedure cells can look the same, they are usually distinguishable by the quantity that are built. If there are less than three they are likely to be supersedure cells, not swarm cells. Swarm cells are often said to be identified by their position on the frame - around the edge, but this is one of beekeeping's many myths. They can be anywhere and often on the edge of holes and gaps.
When queen cells are seen in a colony it is not a good idea to cut any out until you have inspected the colony fully. If a colony is good and the PQN is fairly low, perhaps 10-12, I will try to use the queen cells if I can. This is why I cut out several with a large piece of comb in case I need them later.
There is a view that using swarm cells will perpetuate swarming, but in my opinion this is flawed thinking. Surely it's the genetics that's being perpetuated, not the type of cell the queen came from. I hear the same people advocating making artificial swarms!! I have never understood why it is O.K. to use larvae from a colony for queen rearing purposes, but not a natural queen cell. I have had many really good queens from swarm cells and providing they are not from swarmy colonies they are good to use. They have been built as queen cells from the start and probably under prosperous conditions.