This is a pertinent question in view of the rise of the Internet and electronic communications. In a word, Spam is when someone or some group floods the Internet and people's emails with the same message, in order to gain something from them. Sometimes these messages are to sell something or to gain support. Also often times they are more sinister. They try to get people's personal information and if people give their personal information, there can be identity theft.
The most important point to keep in mind is that these messages are unwanted. The recipients have not signed up for these or even allowed the senders to send them. Therefore, it is a form of invasion of privacy.
There are now many types of Internet filters to can for Spam to limit it.
When something or someone sends messages over and over with a constant pace. Spamming can be harmful since you can freeze someone's computer or mobile device.