Email spam, also known as junk email, refers to unsolicited email messages, usually sent in bulk to a large list of recipients. Spam can be sent by real humans, but more often, it is sent by a botnet, which is a network of computers (bots or spambots) infected with malware and controlled by a single attacking party (bot herder). Apart from email, spam can also be distributed via text messages or social media.

Source: Tech Target