Digimon are creatures that lives in the Digital World
Digimon are monsters that are created within a digital world and are a cross between living creatures and digital information. Their digital universe exists alongside our own normal universe and occasionally are able to cross over and a chosen few humans are able to cross over as well.
Digimon start out in eggs and eventually hatch starting out as Digimon babies. From there they go through a process of evolution, or "digivolution", that allows them to grow and become stronger. When a Digimon gets too old and passes on the digital information is usually reused and turns back into an egg.