Sometimes people trying to SELL you something will say it's a viral marketing system.
Keep in mind that for something to be TRULY viral it must have enough value or be interesting/exciting/fun enough that people WANT to share with other, pass it on,
or tell people to click the link.
One of the most successful viral marketing campaigns ever was ElfYourself by OfficeMax. Tons of people used it and told their friends to go watch the Elf videos they created. Here's a 5 Star thread about it.
I ELFed my Babies & Shawn Collins!
You can't watch the vids anymore - they took them down for the season.