Viral VIdeo

I did not understand this term very well, and I could not find much about the explanation of this term, but it seem the new types of the advertisement for video. One of the website (http://anniva.pekori.to/broad/viral.php) said the Viral Video is to broadcasting the film clip on the Internet which could not broadcast on the TV for some reason. In addition from reading of some website you can find actual Viral Video in some different website like You tube, and this types of website had been popular in very recent. Actually there are some news had shown in recent. I do not know whether it is talking about Viral Video or not, but the film clip which used by Google Video had been used in You Tube without any permission from Google Video which means plagiarism, and You Tube had been indictment.(http://headlines.yahoo.co.jp/hl?a=20061110-00000019-imp-sci)