You want to be able to use your computer for recreation and for work and you want to worry as little as possible about losing data, software crashing and without having some guy from Brazil, Poland or anywhere else in the world hack into your machine and start prying around and maybe deleting your stuff.
This means paying attention to how you set up your computer and the services and software you run on it. In short it means that you have to become your own System Administrator.
Security is: