I noticed that whenever I get an e-mail that's an advertisement, I mark it as spam! What exactly is spam anyway? Wikipedia defines it thus (E-Mail spam):
Definitions of spam usually include the aspects that email is unsolicited and sent in bulk.
A more general definition of spam would be:
Spamming is the abuse of electronic messaging systems to indiscriminately send unsolicited bulk messages.
I guess there would be legal definitions of Spam, and also definitions by ISPs. But as far as the user is concerned, I could safely say that any kind of advertising e-mail that I never wanted in the first place - is Spam. Invitations from random Social networking sites and communities? Also Spam!(?)