Word-of-Mouth

"Word of mouth has, of course, existed forever but is particularly important in social media. The objective is to create content that attracts positive attention and then spreads voluntarily and at no expense to the advertiser. When a commercial or other advertisement goes viral because it is highly creative and people like it, the advertiser wins. An advertiser is less enthusiastic when an advertisement goes viral by virtue of the furore and negative comments it receives."

http://www.blakes.com/english/view.asp?ID=4510