Surf better by using the right fins for your surfboard and the wave conditions. The basic fin selection guide from FCS (click on “Knowledge”) will get you started.
Pros recommend switching out the fins on a board during a single surf session (and in similar conditions) to feel the difference between fins. Basically, smaller fins with less surface area, and fins that are more flexible from side to side create a looser feel. More surface area, and stiffer fins deliver drive and stability. Through experimentation, and with experience, you’ll be able to factor some of the other elements of fin design—like foil and tilt—into your fin selection.
Links to the most popular fin systems are listed below. Check out the pros and cons before buying a new board, and make sure that all of your boards use the same system.
SURF Buyer's Guide 