Disc golf (aka Frisbee Golf) is a recreational and professional sport where the player throws discs/Frisbees to traverse a specialized course in the fewest number of throws. The rules are similar to ball and miniature golf where there is a tee to start from and a goal to reach. There may be obstacles in the way, water hazards and out of bounds areas to add challenge. Goals are typically in the form of chain baskets or light poles, depending on the course.
There are many different kinds of discs spanning various flight paths, speeds and weight for the intermediate and advanced player, but the beginner disc golfer only needs a common household Frisbee to play. The low entree cost to this sport (cost of a Frisbee or disc) and the (mostly) free courses to play on, like Irvine's disc golf courses, make disc golf a wonderful family sport that all age ranges can enjoy together.
Disc golf is also played at the professional level, providing ever-increasing challenges for those who seek it.
For more information about disc golf in general, click this Disc Golf link for the Wikipedia entry.