Here is a nice New York Times video with a basic description of what orienteering is:
Orienteering is a sport in which competitors must navigate through nature using a highly detailed “orienteering” map and a compass. There are checkpoints laid out in the area and they are indicated on the map. Competitors must find these as fast as they can in the specified order. Various variations of orienteering include: night-o, kayak-o, rogaine, mountain bike-o, goat-o, radio-o, score-o, ski-o, trail-o etc. Here are the reasons why I think orienteering is such a great sport, and why it is my sport of choice:
Feel free to check out the Orienteering USA What Is Orienteering? article for a more detailed explanation!