When a computer is in standby mode, it is not completely turned off. Instead, it has already been turned on, but has gone into "sleep" mode. Therefore, when referring to computers, "Sleep" and "Standby" may be used synonymously. A computer in standby mode requires a small amount of current, called a "trickle charge," that keeps the current state of running software saved in the computer's RAM. However, because the computer is in sleep mode, the CPU, video card, and hard drive are not running. Therefore, the computer uses very little power in standby mode.
Since standby mode saves energy, it is a good idea to put your computer to sleep if you are going to be away from it for more than 15 or 20 minutes. You can also use the Power Options control panel (in Windows) or the Energy Saver System Preference (in Mac OS X) to automatically put your computer to sleep after it has been inactive for a specific amount of time. Then, when you take a break from your computer, your computer can take a break as well.