Basically, there are two types of rails: 50/50 rails (which derive from older, pre-shortboard design styles and are found more on longboards) and “down-turned” rails, which can be found on almost all modern surfboards. A 50/50 rail is shaped the way it sounds, meaning the apex, or mid-point, of the curve is in the middle of the rail, creating an egg-like shape that sits high on the water’s surface. Because this type of rail lacks a sharp edge on the bottom, water can more freely flow out from the bottom of the board as you plane down the face of the wave. This buoyancy and a lack of edge make a board with rounder (50/50) rails much harder to maneuver.
A “down-turned” rail is probably what you are used to seeing, as it is the popular choice on most any surfboard today. It begins, essentially, as a 50/50 rail from the nose then at about three-quarters down the rail develops a sharper lower edge to accommodate the flat bottom of the tail area. This sharp edge holds through into the tail and helps to capture the flow of water from the nose and keep it underneath the board so that it creates lift as it runs against the fins in a tighter manner. Down-turned rails are so popular because of the extra maneuverability they give a surfboard. They’re stable yet necessary for vertical, modern surfing.