The main purpose of using swivels in your fishing rigs is to prevent your line from getting twisted. There are different swivels that solve this problem with some being more effective than others. Below is a description of barrel and crane swivels and how they solve the problem.
Barrel swivels are you basic swivels that you can use to attach two fishing lines together. They are the most inexpensive type of swivel therefor they are very popular among fishermen. Barrel swivels are somewhat limited when it comes to preventing line twists. Because of their design there is a lot of friction and they do not spin well when under pressure. That's why they should be used when you want a cheap swivel that attaches your line and line twisting is not too much of a concern.
Both barrel and crane swivels can be a snap swivel. A snap swivel comes with a snapping attachment at one end. This makes it easy for an angler to quickly swap between different lures. If you're going to swap between different lures often then a snap swivel is the right solution for you