A virus writes itself to disk, needs access to CPU  on the computer and attaches itself to other files.

It has three functional parts, the combination of these makes the program a virus:

Hoaxes: Also virus

Two different types:

1. Operates on one single computer, like a normal application or program.
2. A central node on one computer, but different segments of the worm program on other computers in a network. (Octopus)

Is a self-supporting program and replicate itself via an open line to the internet (dial-up or permanent).

Performs an activity that the user is unaware of.  A hidden application within a program.

Are not viruses because they do not replicate and attach themselves to other files.