All Digimons are hatched from a Digitama Egg, go through a baby Digimon phase, and grow into In-training,Rookie, Champion, Ultimate and Mega form. What a Digimon needs for these steps is Data.
Basically, Digimon hunt other Digimon, load (absorb) their data to grow, and move on to the next Digivolution step once they grow beyond a certain level.

But the Digimon that have a Partnership with humans can be forced to grow through the power of each human partner. These partners are called Tamers, and a Digimon that has a Tamer is called a Partner Digimon.

Digimon Masters skips the DigiEgg -> Infancy -> In-training part, and players start with Digimon in their Rookie. Some of the in-game characters can go beyond Mega to Burst Mode.