Under the Spam Act 2003 it is illegal to send, or cause to be sent, unsolicited commercial electronic messages. Complete info can be found on the ACMA (The Australian Communications and Media Authority) site.
Basically Spam is email (or also SMS message nowadays) that you did not request or don't want, they are the likes of the nasty slimming or 'enhancing' pills, our mates with Nigerian Scams, basically anything asking you for money that is not legitimate. Some businesses are not aware of this, but just sending an email to another business promoting your services when it was not requested can also be considered Spam.
There are all sorts of programmes out there, most of them free like Spam Assassin, Barracuda and more that can be installed on your computer to stop Spam, though be very aware that if you are not terribly Net savvy, they can be more harmful then helpful as they are just software, which by nature is dumb, and can delete all sorts of mail that you do really want. Unless you are getting 100's of spam email per day you are best to just use your trusty little delete finger, or you may lose that important email you were waiting for. Many email programmes have in-built spam software, so you may have a 'junk' or 'spam' folder, always have a look there if you are missing email you are expecting, as of course your software always thinks it knows best or better then you anyhow...
If you do get an awful lot of spam, then call in an IT expert to help you.