Rocker is the amount of curve that the surfboard has from the nose to the tail. The amount and variation of rockers determines how your surfboard will turn. This means that the more rocker, the easier the board will turn but the less speed and drive it will have. Flatter waves suit a flatter rocker while hollow, bowly surf suits more rocker.
Thickness of a surfboard determines the buoyancy it will have. The more buoyant the easier it will be to paddle but the more unresponsive and sluggish it will be. The thickness of a surfboard is measured to your weight and level of surfing.