The biggest security risks occur if your Mac is stolen or physically compromised. Even if thieves can't log into your account, they can gain access to the data on your Mac using one of the many special start-up modes built into all Macs, such as booting from an install DVD and resetting your password, using Target Disk Mode to make your Mac act as an external hard drive, or booting into the Unix-style Single User Mode.
You can, however, place a firmware password on your Mac. This password is written into the firmware chips on the Mac's motherboard using either the Open Firmware standard on PowerPC Macs or Extensible Firmware Interface (EFI) on Intel Macs. Regardless of platform, the free tool from Apple for implementing a firmware password is called the Open Firmware Password Utility. Apple provides complete steps for setting a firmware password on its support site.
If you or anyone else tries to use a special start-up mode, the user will be required to enter the firmware password. This can significantly secure personal, business or educational Macs against tampering. However, be warned that if you forget a firmware password, there is no way to reset or remove it