What are the characteristics of a computer virus?

Replication mechanism, activation mechanism and objective.

Replication mechanism: To replicate a virus needs to attach itself to the right code, where it can replicate and spread past security systems into other systems. Activation mechanism: Most viruses require the user to actually do something. During the 80s and early 90s most viruses were activated when you booted from a floppy disk, or inserted a new floppy disk into an infected drive. Nowadays most computer viruss come as email forwards, and they require the user to execute. Objective: many viruses have no objective at all, but some have the objective to delete data, hog up memory, or crash the system.