Fish traps are all pretty similar in theory. Each one has a holding space and an entrance usually equipped with some sort of barbs or other form of exiting resistance. A fish will enter the trap at the entrance hole. This point becomes narrower and narrower until the fish must squeeze through the opening into the holding space. After the fish enters the trap it will be unable to escape because it is one thing to swim past the barbs, but another thing entirely to swim against it. It is trapped.
I like this design because it is easy to carry, super cheap and easy, has a large holding space, and the lid is interchangeable depending on the fish you are after (i.e. catfish or crayfish).