It was produced last year, but this report from the US-based Center for Democracy & Technology is an excellent plain English explanation of why you get spam; how the spammers acquire your email address; and how to reduce the chance of receiving unsolicited email.
“In the summer of 2002, CDT embarked on a project to attempt to determine the source of spam. To do so, we set up hundreds of different e-mail addresses, used them for a single purpose, and then waited six months to see what kind of mail those addresses were receiving. It should come as no surprise to most e-mail users that many of the addresses CDT created for this study attracted spam, but it is very interesting to see the different ways that e-mail addresses attracted spam — and the different volumes — depending on where the e-mail addresses were used. “