Spam, or more formally unsolicited commercial email, is an unrequested promotional message that gets sent to millions of email addresses at a time, usually to advertise a product or service for sale over the Internet, or to recruit people for some scheme.
Offline, we may be used at receiving unrequested printed advertising in our mailbox and throwing it away without caring too much. Online, however, the sheer scale of the issue becomes a huge problem: as any commercial enterprise can send millions of emails throughout the world in a short time and at a very low cost, our mailboxes end up being flooded by advertising, while the actual cost of this practice is borne by the users and by their Internet service providers.
This is why nowadays no reputable business would use spam to promote their legitimate offer, and spam is mostly used for scams and illegal concerns (for example unlicensed pharmaceutical trade).
Reputable businesses only send you advertising when you accepted it in advance, for example as part of the conditions for receiving some free service, or by ticking a box in an online registration form; the practice of opt-in (an explicit, voluntary and optional acceptance of the use of your email address for future promotional messages) is required by the law in the European Union and in many other countries.
