He/She who runs ultramarathons is usually considered an ultrarunner.
An ultramarathon or an ultra, is any race involving running longer than the traditional marathon length of 42.195 kilometres (26.2 miles).
There are two types of ultramarathon events: those that cover a specified distance, and events that take place during a 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 24/48-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 often on 400-meter tracks, 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, especially in the US are run on dirt roads or mountain paths, popularly known as trail running 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, 6 and 10 days - known as multidays.These events are generally run on a track or a short road course, often one mile or less.
The most popular 
form of multiday races are the stage races such as the Marathon des Sables which takes place every year in the Moroccan desert. The MdS is a 6 day stage race covering 254 km (156 miles).
In 2011 There will be a Trans-American race from LA to New York organised by French ultrarunner Serge Girard starting June 19th and lasting for 70 days.
Timed events present a slightly different challenge to runners as the runner has to create his own schedule in order to reduce non-running time to a minimum. The 6 day events are standard events that have a history dating back to the 1880's and in the ultrarunning rennaisance that began in the 1980's the re-establishment of annual 6 day races has inspired a whole new generation of ultrarunners to step up to the multiday challenge. Primary among these races are the Self-Transcendence 6 & 10 day races that take place each April in New York on a one mile loop in Flushing Meadow Park.
Multidays.com is a popular source of race information for multiday races and ultramarathons as well as many links to ultramarathon sites, ultrarunning resources and also to ultrarunners and other international news sites.
Ultrarunning World is a sister site to Multidays.com and publishes a magazine featuring race reports, news and a comprehensive ultramarathon calendar.
Ref:
Wikipedia.org - Ultramarathon
