Windsurfing is a surface water sport using a windsurf board, also commonly called a sailboard, usually two to five meters long and powered by a single sail. The rig is connected to the board by a free-rotating flexible joint called the Universal Joint (U-Joint). Unlike a rudder-steered sailboat, a windsurfer is steered by the tilting and rotating of the mast and sail as well as tilting and carving the board. This method of controlling the board's direction is called the free-sail system.

The sport combines aspects of both sailing and surfing, along with certain athletic aspects shared with other board sports like skateboarding, snowboarding, waterskiing, wakeboarding and kitesurfing. Although it might be considered a minimalistic version of a sailboat, a windsurfer offers experiences that are outside the scope of most other sailing craft design. A windsurfer holds the world speed record for sailing craft (see below); and, windsurfers can perform jumps, inverted loops, spinning maneuvers, and other "freestyle" moves that cannot be matched by any sailboat. Windsurfers were the first to ride the world's largest waves, such as Jaws on the island of Maui, and, with very few exceptions, it was not until the advent of tow-in surfing that waves of that size became accessible to surfers.

Windsurfing includes speed sailing, slalom, course racing, wave sailing, superX, and freestyle as distinct disciplines.

Though windsurfing is possible in winds from near 0 to 50 knots, the ideal planing conditions for most recreational sailors is 15-25 knots, with lighter winds resulting in displacement mode sailing.

Lessons can be taken with a school. With coaching and favorable conditions, the basic skills of sailing, steering, and turning can be learned within a few hours. Competence in the sport and mastery of more advanced maneuvers such as planing, carve gybing (turning downwind at speed), water starting, jumping, and more advanced moves can require lengthy practice.

Training DVDs exist which are useful in a sport where it is difficult for a coach to be close to a pupil particularly when learning the more advanced manoeuvres.