Di, a viral campaign is basically a video, flash game, animation, or anything of the sort that has great social networking potential. It's something that people are going to talk about and pass on, and a truly successful viral campaign spreads within a couple of days max and minutes or hours minimum since being launched. It may seem unrelated to the actual brand or product, but increases brand awareness via its popularity. Often viral videos aren't tied to ANY brand and sometimes companies buy rights to existing viral videos to profit from them (McDonald's did this I believe...) Here are some examples of videos that went viral:
Leave Britney Alone: YouTube - LEAVE BRITNEY ALONE!
This video went viral and made Chris Crocker's popularity shoot through the roof. It's been "remixed" countless times and got Crocker numerous appearances on television shows.
The Free Hugs Campaign: YouTube - Free Hugs Campaign - Official Page (music by Sick Puppies.net )
The Free Hugs Campaign video is still one of the most watched YouTube videos of all time.
LOLcats: Lolcats, tagging funny/stupid captions onto photos of cats, are a viral craze.
Numa Numa Video: YouTube - Numa Numa
This Numa Numa video spread through the internet and burned the face of this guy into internet history. He went on to try to profit from the craze by making another Numa Numa video, but the second video obviously had production money put in and just doesn't have "it" like the original.