Legal advertising == Spam ?

There was recently a story on Slashdot, about a court case - e360 Vs. Comcast - in which the judge declared e360 a spammer. e360 calls itself an Internet marketing company.

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!(?)