A viral video is usually a short clip or section of film. Typically it will be posted on the internet by a member of the public, though it has sometimes been done for business and advertising purposes too. The video will appear on a networking site or a video sharing site such as YouTube, or sometimes may be sent by email. The video becomes viral when, for whatever reason, it gets so popular that thousands, and eventually millions, of people click on it, and often transmit it further by sending a link to it to their friends.

A lot of new viral videos appear every day. You can even find the ones considered most watchable by clicking on Mashable's viral video of the day page or putting a phrase like 'top ten viral videos' in a search engine.

Most viral videos are posted because people find them funny or unusual - like Pudsey the dancing dog.