Spam is the use of electronic messaging systems (including most broadcast media, digital delivery systems) to send unsolicited bulk messages indiscriminately. While the most widely recognized form of spam is e-mail spam, the term is applied to similar abuses in other media: instant messaging spam, Usenet newsgroup spam, Web search engine spam, spam in blogs, wiki spam, online classified ads spam, mobile phone messaging spam, Internet forum spam, junk fax transmissions, social networking spam, television advertising and file sharing network spam.
Spamming remains economically viable because advertisers have no operating costs beyond the management of their mailing lists, and it is difficult to hold senders accountable for their mass mailings. Because the barrier to entry is so low, spammers are numerous, and the volume of unsolicited mail has become very high. In the year 2011, the estimated figure for spam messages is around seven trillion. The costs, such as lost productivity and fraud, are borne by the public and by Internet service providers, which have been forced to add extra capacity to cope with the deluge. Spamming has been the subject of legislation in many jurisdictions.
A person who creates electronic spam is called a spammer.
Email spam, also known as unsolicited bulk Email (UBE), junk mail, or unsolicited commercial email (UCE), is the practice of sending unwanted email messages, frequently with commercial content, in large quantities to an indiscriminate set of recipients. Spam in email started to become a problem when the Internet was opened up to the general public in the mid-1990s. It grew exponentially over the following years, and today composes some 80 to 85% of all the email in the world, by a "conservative estimate". Pressure to make email spam illegal has been successful in some jurisdictions, but less so in others. Spammers take advantage of this fact, and frequently outsource parts of their operations to countries where spamming will not get them into legal trouble.
Increasingly, email spam today is sent via "zombie networks", networks of virus- or worm-infected personal computers in homes and offices around the globe; many modern worms install a backdoor which allows the spammer access to the computer and use it for malicious purposes. This complicates attempts to control the spread of spam, as in many cases the spam doesn't obviously originate from the spammer. In November 2008 an ISP, McColo, which was providing service to botnet operators, was depeered and spam dropped 50%-75% Internet-wide. At the same time, it is becoming clear that malware authors, spammers, and phishers are learning from each other, and possibly forming various kinds of partnerships.
An industry of email address harvesting is dedicated to collecting email addresses and selling compiled databases. Some of these address harvesting approaches rely on users not reading the fine print of agreements, resulting in them agreeing to send messages indiscriminately to their contacts. This is a common approach in social networking spam such as that generated by the social networking site Quechup.
Instant Messaging spam makes use of instant messaging systems. Although less ubiquitous than its e-mail counterpart, according to a report from Ferris Research, 500 million spam IMs were sent in 2003, twice the level of 2002. As instant messaging tends to not be blocked by firewalls, it is an especially useful channel for spammers.