An ultra marathon is any race, on foot, that is longer than the 26.2 mile traditional marathon. Although there is no limit as to how long an ultra marathon can be, the most frequent starting distance is 50 kilometers.

The longest ultra marathon ever held was the Ultimate Ultra. Other long races include the Sri Chinmoy and the Trans America Ultra. These races can be run on roads, tracks, or trails, and can be point to point or take place on a continuous loop.

If you plan on running an ultra marathon for the first time, there are some key training differences to keep in mind. First, your pace will be much slower and you will find less of a need for speed work. Second, you will need to run more miles per week than when training for a traditional marathon. Third, you will eat a lot more not just while training for, but also while competing in, an ultra marathon (think water stations with pizza and candy!). Finally, you will have to wear a hydration pack with plenty of storage for food and other items, as many ultra marathons don't have as many water or aid stations as you would think.

Remember- ultra marathon running isn't about speed. The endurance is what is important!

http://www.ultrunr.com/what_is.html
http://blog.theclymb.com/out-there/4-primary-differences-between-marathon-and-ultra-marathon-training/