A petition is a document, written most often by an interest group, with a list of demands or statements written on it. Underneath the statement is space for signatures gathered from anyone who agrees with the statement. It is then sent to the person or organization which it is attempting to pressure, with the intent to show mass public opinion on the subject.
Online petitions are becoming increasingly popular. The British government has an e-petition site, and any petition attracting more than a certain number of signatures is guaranteed a discussion (though not a vote) in the House of Commons. However petitions distributed by chain email are worthless, as multiple copies quickly appear and it is impossible to get an actual count of the names.