this is when a couple or gang of lads/lasses attack and beat someone up while another person usually films it on a camera phone and then sends it to all other friends etc.etc
The phrase 'happy slapping' is a media invention to dsecribe an isolated incident, which has since grown in 'popularity', probably as a result of the press attention it has received.
i can tell you what a happy slap is, its where you plan 2 slap hit or even punch somebody and record it on your video phone, some times it can get out of hand and even cause serious injuries to the victim.