As mentioned in the introduction to this section, electronic mail is old by Internet standards and is now used in situations that were never even dreamt of when it was originally devised.
In most ways it has aged well and has adapted to allow many things that it was never originally designed to do, such as attaching files to messages, support for non-English languages and encrypted messages to name just a few.
In other ways, it hasn't fared quite so well, the most obvious being junk mail or "spam". While there were only a few, well managed systems attached to the Internet, controlling email abuse was easy ("Don't do it again or you will lose your access"), but as it became bigger, control became harder and the abuse increased.
In simple terms, junk mail is anything that appears in your inbox that you didn't ask for and don't want.
When junk mail started to become a nuisance (sometime in the late 1990s) a lot of it was just advertisements ("You don't know us, but we sell product X and we want to tell you about it"). They were annoying, but manageable.
Nowadays, junk mail is a major problem. Partly, this is due to the sheer amount of it (it was estimated in 2007 that over 90% of all email received was junk), but also because the content of these messages has changed: the simple, but annoying adverts of the 1990s have been replaced by an assortment of scams, computer viruses and attempts at credit card theft.
It might sound obvious, but the best way to reduce junk mail is not to publicise your email address. Sometimes this is out of your control (there have been viruses that collect email addresses from the address books of infected machines), but there are also times when you just need to be careful (why does that web site want to know your email address before it shows you the amusing animation?)
If you are lucky, your email provider will have some form of filtering that you can use. If they do not, or you want more protection, you could run a local filter on your PC (there are various commercial solutions for Windows and Macs and quite a few free ones for Linux and other Unix-like systems). You need to be aware, that filtering isn't foolproof. Some junk mail will get through undetected and some legitimate mail will get misidentified as junk.