Skimboarding is a relatively new sport, in mainstream terms. It has been around, in one form or another since the 1950's, when people cut pieces of plywood into surfboard shapes. Skimboarding involves the use of a flat board, made of either wood or fiberglass, and a nice long stretch of beach. When the waves wash up onto the shore (best when the tide is going out), a thin film of water is left as the wave pulls away.
Right as this film forms, you take your skimboard, and, making a shoveling motion, slide the board parrallel to the sand/water film. When the board makes contact with the thin film of water, it starts to slide. At this point, you run as fast as you can to catch up with the board, and hop on, both feet at once. You then attempt to maintain your balance, and ride out the wave as long as you can.
There are several tricks you can do while riding the wave, from simple spins to "kick-outs" (Kicking the board out from under your feet to allow it to pick up speed again), to headstands, etc. You can also aim the board toward the deeper water to jump the waves as they come in, as long as your board is sturdy enough.
Boards are generally cheap, ranging from $30 dollars (for wooden or used boards) to $200 (for high-quality fiberglass boards). Without a high entry price, skimboarding is a sport that is relatively easy to get into, and a great way to spend time at the beach.