Twin + 1 Surf Fins
A twin setup is usually found under retro boards. This fin setup makes your board very loose and manoeuvrable. Twin fins have two large fins at the side of the bottom of a board, which are foiled. This gives your board sidewards pressure and more grip on the rails. This is why boards with a short rail line, or very round boards with little effective rail, usually have a twin fin setup. This setup gives the board more grip. Besides, the absence of a fin in the middle, enables the water to flow through easily. This allows more speeds, though decreases the amount of control.
A twin + 1 setup does have a smaller fin in the middle of the board. This setup gives the board more control, but decreases its speed. The center fin is available in different shapes and sizes, which have a different influence. Check out, for example, the twin+1 fin sets from FCS, Future fins and Captain Fin!
Check the Ultimate Surfboard Fin Guide for more information!