from WidipediA: - An ultramarathon (also called ultra distance) is any sporting event involving running longer than the traditional marathon length of 42.195 kilometres (26.2188 mi).
There are two types of ultramarathon events: those that cover a specified distance, and events that take place during specified time (with the winner covering the most distance in that time). The most common distances are 50 kilometres (31.069 mi), 100 kilometres (62.137 mi), 50 miles (80.467 km) and 100 miles (160.934 km), although many races have other distances. The 100 kilometers is an official IAAF world record event.
Other distances/times include double marathons, 24-hour races, and multiday races of 1000 miles or even longer. The format of these events and the courses vary, ranging from single or multiple loops (some as short as a 400-meter track), to point-to-point road or trail races, to cross-country rogaines. Many ultramarathons, especially trail challenges, have severe course obstacles, such as inclement weather, elevation change, or rugged terrain. Many of these races are run on dirt roads or mountain paths, though some are run on paved roads as well. Usually, there are aid stations every 5 to 15 km apart, where runners can replenish food and drink supplies or take a short break.
Timed events range from 6, 12, and 24 hours to 3 and 6 days (known as multi-day events). Timed events are generally run on a track or a short road course, often one mile or less.
The International Association of Ultrarunners (IAU) organises the World Championships for various ultramarathon distances, including 50 km, 100 km, 24 hours and ultra trail running. These events are sanctioned by the International Association of Athletics Federations (IAAF), the world governing body of track and field. Many countries around the world have their own ultrarunning organizations, often the national athletics federation of that country, or are sanctioned by such national athletics organizations. World records for distances, times and ages are tracked by the IAU.