What is spam?

Spam doesn't have a single definition, and the word has more than one meaning. However, the one most recognisable is that of unwanted e-mails. If looked up in the dictionary, it is usually the only definition, next to the meaning of canned meat (and these are somehow related to each other- for more deatails see History of spam post).
E-mail spam was one of the first types of spam that appeared in the internet. The Internet is developing fast, and as it can be seen on Wikipedia, electronic spam has now many subcategories.
In general, spam is an "agressive" message (one that is finding its way to a receiver, rather than waiting to be received), often called unsolicited. It is usually a bulk message (one sent to many recipients at the same time), and very often repetitive- that is, one user can receive it multiple times.
As the Internet develops (discussion boards, instant messaging, online games etc.), the range of methods to send this kind of messages increases. However, the topic is way too broad to describe every single type of spam here, and because of that the type of spam I'm going to focuse on is that sent by e-mail.
If you wanted to know more about different type of spam, I recommend the Wikipedia site I've reffered to before (electronic spam), it gives a good overview and a base for further research (as it was for mine about e-mail spam).