In essence, it is viral marketing, with a difference, the difference being that if you can affect 1 individual by your marketing, you can in essence affect dozens more.
Its actually a relatively new 'term' that was created late last year by a couple of people and it has kinda grown, in that more people are using the term than they were this time last year LOL
As i say, in essence it is just a new word for viral marketing via word-of-mouth advertising - at least in theory
Regards,
Lee