Your vote on this answer has already been received
Spam is any message or posting, regardless of its content, that is sent to multiple recipients who have not specifically requested the message. Spam can also be multiple postings of the same message to newsgroups or list servers that aren't related to the topic of the message. Other common terms for spam include UCE (unsolicited commercial email) and UBE (unsolicited bulk email).
The individuals who send spam are typically people who purchased or harvested a list of email addresses. They send messages from numerous different addresses to all areas of the Web. These messages tend to be "forged," to hide who actually sent them.