Spam is a term for unsolicited mail sent via e-mail message. Spam often contains marketing messages and offers, but spam messages may also contain viruses or hidden programs to obtain personal information (spyware) or other deceptive techniques such as phishing, pharming, and other malicious software.
Spam is sent out to email addresses that were obtained mostly collectively - either by buying them from an infected computer contacts (for example using spyware) or by scanning the websites by spambots.
Spam in practice: Protection against spam is provided by anti-spam programs. Anti-spam protection is usually multi-layer. The first layer is provided by internet service providers (ISPs), another layer of protection against spam is at the email servers, and the last layer is on the end device. The protection against spam is never foolproof.