It is important to prevent others from using your computer when you step away. You must know your login ID and password before trying of any of these options.
Please note the following item from the Information Security Policy, "the maximum idle time before the screen saver invokes is 15 minutes."
Use Screen Saver Password Protection
MAC X 10.4 and Higher
Use Screen Saver Password Protection (Desktop & Screen Saver)
Use Screen Saver Password Protection (Security)