David Pogue's book will go a long way toward removing the mysteries.
The Mac is pretty forgiving and won't let you cause any permanent damage, however, nothing can stop you from erasing all your photos or music or whatever you have on your Mac that you would prefer not to lose. Even if you don't do something dumb, hard disks have a finite life and eventually fail, often with no warning. Most people don't realize how much they have on their computer or how valuable it is until they lose it all.
Don't be one of them. Read this: Mac 101: Time Machine
You have to buy an external hard disk but they are not terribly expensive - a little over $100. Configuring it is literally as easy as plugging it in. Time Machine asks you if you want to use it as a backup, after which it does the rest completely on its own.
Time Machine is the best way to avoid the consequences of messing something up, and is worth it for peace of mind alone.