The main distinguishing factor between a legitimate message and Spam is consent. Simply put, if you asked for it, it is not Spam. As an example mass mailings of e-mails of a commercial nature are legitimate if you invited the communiation by signing up for "news" on certain topics or for offers of a particular kind.
E-mail from friends is not Spam. Receiving forwarded mails from friends although annoying, is not Spam. If the sender is known to you, the best way to put a stop to it is to politely ask them to stop sending you such mails.