Fins provide a large surface area so your powerful leg muscles can propel you through the water which is much more efficient than swimming with your arms.
There are divers with limited leg use who can use specially designed hand fins and their arms to propel them through the water.
There are plenty of different fin designs on the market, but the two main types you need to be aware of are the full foot and open heel fins. It isn't hard with their names to understand the difference in appearance, but the important thing is to understand the difference in purpose.
Open heel fins, also referred to as adjustable fins, are much better for divers and generally preferred by both beginners and professionals, especially in colder waters, as they allow you to wear dive boots and still fit the fins over easily and efficiently. The full foot fins are more suited to warm water as you don't need to wear dive boots with them; convenient and popular for beginners, holiday makers and casual snorkelers/divers that do not require the extra insulation allowed by the open heel fins.