What is this viral advertising?

Viral advertising is a type of advertising that uses existing infrastructure to distribute retinal specific brand. What that may be learned about the publication of a mouth to ear now getting a slightly different meaning in the era of the Internet with the use of email recently with the increasing use by social networks.
Probably most of us are exposed every day to this form of advertising - even if we are not aware of this: Yes, it is all those emails that everyone just love pass each other all the time at work and contain without we know " underground" advertisements. it could even be a blog that we really like to read that behind it had a campaign, and that "blogger" we love his writing and some rely on his word actually "sell" us through it.

I have just heard about viral marketing
Till now I don't know how to do that on facebooke