A warm boot (also called a "soft boot") is the process of restarting a computer. It may be used in contrast to a cold boot, which refers to starting up a computer that has been turned off.
Warm boots are typically initiated by a "Restart" command in the operating system. For example, to perform a warm boot on a Windows system, select from the Start Menu. If you use a Mac, you can perform a warm boot by selecting from the Apple Menu.
Warm booting (restarting a computer) is more common than cold booting since most people leave their computers in sleep mode when they are not using them. While a home computer may not need to be turned off for months, it may need to be restarted every few days or weeks to complete new software installations.
There should be no difference in the way a computer performs after starting from a warm boot vs a cold boot. In both cases, the computer must complete the full boot sequence. This process loads all system files, including any that were installed immediately before the computer was restarted. The only difference is that some machines may perform a more complete power-on self-test (POST) at the beginning of a cold boot.