Fins prevent squirreling and keep the board going in the direction it is pointed. By creating drag, the fins pull down the tail of the board to track in the water. The shape and size of the fin will have a huge impact in how it performs. A large fin creates more drag than a smaller fin because it is digging into the water more, making the board more stable. A shallower fin will allow for quicker reaction times and sharper turns.
The most important thing to consider is the fin configuration. There are singles (one center fin on both ends), quads (four fins, with two on each end and no center fin), six fins, and molded-in fins (with a removable center fin).
Some riders prefer to choose their fins based on water conditions. If so, consider that in rough water, a larger fin gives the rider more traction and control in the water. In smooth water, the freedom of movement of a smaller, shallower fin is preferred by most riders.
The fastest fins are made of G-10. Since the material is so structurally strong, the fins can be smaller without flexing. Fins are also made of polyester, which provide a steady and predictable ride.