
Source: What is Spam?
Spam is flooding the Internet with many copies of the same message, in an attempt to force the message on people who would not otherwise choose to receive it. Most spam is commercial advertising, often for dubious products, get-rich-quick schemes, or semi-legal services. If you didn't signup for that mailing list, or give authority to sell your e-mail address, then your address was received unlawfully and it's considered spam.
E-mail spam targets individual users with direct mail messages. E-mail spam lists are often created by scanning forum postings, guestbooks, stealing Internet mailing lists, or searching the Web for addresses.
One particularly nasty variant of e-mail spam is sending spam to mailing lists (public or private e-mail discussion forums.) Because many mailing lists limit activity to their subscribers, spammers will use automated tools to subscribe to as many mailing lists as possible, so that they can grab the lists of addresses, or use the mailing list as a direct target for their attacks.