Digimon (デジモン, Dejimon), short for Digital Monster (デジタルモンスター, Dejitaru Monsutā), are mysterious lifeforms that are the main focus of the eponymous Digimon franchise. The vast majority of Digimon reside within the Digital World, a region of pseudo-cyberspace that was created on the Network, and originated from the Net Ocean, the birthplace of life within the Digital World. There are many different species of Digimon, some of which have their own subspecies, as well as unconfirmed and new generations that have yet to be revealed.
One day, a virus possessing artificial intelligence spread onto computers throughout the world. This virus evolved in a way similar to living organisms, changing its appearance and nature on the Network, becoming the "digital lifeforms" that came to be known as "Digital Monsters", which were first discovered on there in 1997.
Digimon evolve with age, collecting data, winning battles, or drawing energy off a human. Digimon have six "natural" stages of evolution as well as others obtained through artificial help.
While the anime generally presents Digimon as having conceptually "matching" evolution lines, in other aspects of the franchise, Digimon evolve depending on how they're raised - including what time they're awake, how much they've fought, how much they've trained, how much they've eaten, and other factors. An Agumon who has been raised properly will evolve into the powerful Greymon, while one who has been starved to death will evolve into the weak and disgusting Numemon. Evolution can be temporary or permanent depending on the circumstances. Generally in the anime, Digimon evolve for a short period of time before reverting to smaller, cuter forms. In the games, Digimon usually retain their larger forms after evolving.