Spam mail is generally any form of unsolicited mail that is received from an unrecognized sender. Spam is quite often sent in bulk to several e-mail recipients and is usually intended to advertise merchandise and elicit sales.
People may use spam as a blanket term to describe all types of unwanted e-mail communication, but this is not technically accurate. Some people sign up for mailing lists and forget, calling the messages received from these companies spam. The definition of spam seems to hinge on the recipient's reaction to the mail. In some cases, people are receptive to unsolicited messages that are targeted to their demographic.