The Digimon Branch is made up of multiple worlds with a few things in common. First, the world in which humans live tends to be Hub-like or similar with only slightly higher technology than the actual hub at a similar time frame.
Second is the fact that these Hub-like worlds are connected to an alternate plane through technology, the Digital World. The Inhabitants of the Digital world are made of computer data, often files that were partially corrupted or lost. Depending on the world, dreams, wishes, and desire to live can also play an important function.
Third, it is possible for humans to enter the Digital World--and usually the opposite holds true, but not always.
Forth, there is a better than average chance that travel to the Digital World will involve some sort of Time Dilation effect.
Fifth, it is possible for Digimon and humans to cooperate together and form a symbiotic partnership. The Digimon gains extra strength and potentially other abilities in times of danger, the human gets greater safety in dangerous situations, and both get companionship. Often, it seems Digimon will tend to partner with children and teenagers, though partnerships forming during adulthood are not unknown. These Partnerships seem to be for life.