E-mail spam represents an unsolicited message, usually (but not always) of a commercial nature, indiscriminately sent to you and to a large number of other recipients by an unknown sender.
If you are reading these lines, you are probably one of the 1,407,724,920 people from this planet, according to a statistic published couple months ago, who own a computer and get online quite often. Chances are that you received this article or a link directing you here within an e-mail. And, most likely, this should also be true for the other several readers laying their eyes on the same paragraph by now.
You probably noticed some of them, by their names and e-mail addresses appearing next to yours in the header of the message I just mentioned. And, almost certainly, at least one or two guys from the To field are total strangers for you, if not all of them (not to mention the name appearing in the From field which, by the way, is not mine). There is also a very good chance for you not to be interested at all in receiving and reading this article, in which case I must humbly apologize in advance for wasting your time and patience, and respectfully assure you that this was not in any way my intention.
Technically speaking, an e-mail message bearing the features I outlined in the previous paragraph is called spam or bulk e-mail or just junk e-mail. An utterly unpleasant (post-)modern times invention that you, and me, and one fifth of the (Internet connected) world