They are used to make the vocalizations of the hen. I'm no expert by far. I have a friend that can make a call sing! He can make clucks, purrs, yelps, all the stuff that a hen does to excite the boys! There is no single call that does it all. You've got mouth calls, pot calls, box calls, trumpet calls and probably a dozen I've forgotten. I know with pot calls there's a blue million combinations of wood and surface that can make the sound different. Even what the striker is made of can make a huge difference in the sounds that the pot can make. The striker is a stick of sorts that the tip is used to rub across the top. The top surface can change the sound. You can have slate, glass, crystal, aluminum, copper, stainless etc. The distance between the top surface and the soundboard which sits underneath the top can also change the sound, and you guessed it you can have different things for the soundboard. I guess like anything people do you start out with something that looks so mundane and then you realize the details can be crazy! Of course different people have their preferences as to which combinations they like best. They are fun to make and don't take much wood, so you can use some exotics. You only need a 4x4x1 inch piece plus the sound board and the top surface. Like anything, I've seen them go for well over 100 bucks! It's all about the name! I've sold several in the 30 dollar range. Usually I just sink it back into wood and parts.