Safe Boot is used for both diagnostic reasons, and to actually solve problems. Be sure and restart AGAIN after performing a Safe Boot or your computer will run in a crippled mode until you do.
To perform a Safe Boot, start up (or restart) your Macintosh computer while holding the SHIFT key. In OS X 10.6.8 and later, look for a progress bar on the screen. Once you see it, let go of the shift key. You will be asked to login with your user name and password. Be sure you know both of them before you try a safe boot.
If you are running an older operating system, you won't get a progress bar at all. Just hold the shift key until you are asked to login.
Once the Safe Boot is complete, your screen will look normal (or almost normal). Restart to use your computer.