As the name suggests, virus types are 'corrupted code' something has gone awry and what has come of it is a creature manifesting the corruption. Just like dark types, virus types are not always evil, though they have a leaning towards more villainous natures.

purples and deep blues, sharp teeth, feral goblins, and villainy are all pretty standard virus traits.

Vaccine

Vaccine is the inverse, code created to heal or help virus code. A repair kit if you will, vaccine types are traditionally holy, angelic, healing spirits. Vaccine are often associated with the color yellow, and are often depicted in golds and whites.

Often vaccine types are over the top good, sweet, or naive.

Variable

Variable data is data that is unstable, but not necissarily corrupted. Often variable digimon are lower in the evolution stage, and it represents the great deal of potential there is in their evolution. Variable digimon are rare and often more unique variations on their themes.

Free

Free is the data before data, old code that has withstood the test of time and remained, unbound by the logic and function of modern codes, free is by far the most unique and self holding type. "Fresh"/"Baby" Digimon are also almost always 'free' as they have yet to become a specific type of code, and remain unbound, all around, free.