Viral refers to the spread of information, most notably over the internet, through person-to-person interactions. It is closely related to memes, which are often the unit of information being passed. The exposure patterns to the information look very much like the spread of a biological virus or other infectious agent. It is essentially an exponential growth pattern, with one person showing it to a few friends, who each show it to a few more friends, and so on.
The ease of spreading information in this manner over the internet has led it being the main fertile breeding ground, but similar things can occur through older technologies like fax machines, smoke signals, or, if desperate, face-to-face communication.
In recent years, major corporations have attempted to create artificial viral marketing campaigns to take advantage of the low cost and high exposure. Such viral marketing campaigns have met with mixed success.