The "sleep" and "hibernate" functions in laptops serve similar purposes, but they are different processes and the computer goes through different steps depending on which is activated. Generally, sleep is more useful when the laptop still has access to a continual power source, while hibernate is useful when the laptop needs to conserve energy.
Both the sleep and the hibernate systems available in laptops (especially those with Windows operating systems that have these functions built in) are used when the computer needs to be shut down but will be used again soon. You can activate one of the modes to secure the computer from outside access or to help save power and enable a quick restart. These modes are often automatically activated when the laptop is closed, depending on how it is programmed.
Sleep, also known as standby, is a state in which the computer shuts down nonessential systems but leaves the basic components of the system turned on. All the data in the physical memory of the computer is still kept in the internal memory, ready to be used, but all other systems and internal processes are halted. Usually, sleep mode is indicated by a blinking light on the laptop.
Hibernate is a more long-term mode that's best to activate when the laptop will not be opened again for some time. Instead of saving current data in internal memory, the computer creates a file that stores what it was doing before it hibernated, then saves that data in the hard drive. When the computer is turned on again, it opens that file and restores all systems to the way they were.
Hibernate shuts down all computer systems and is useful for saving power. Sleep mode keeps some systems on and consumes power continually -- if the power runs out, then sleep mode deactivates and the computer shuts down completely. A laptop plugged into an outlet can be put into sleep mode indefinitely, but a laptop on battery power should be put into hibernation to conserve the battery.
Both sleep and hibernate modes restore computer functions more quickly than booting the computer up from a complete shut down. However, sleep immediately accesses internal memory that the computer has kept. Hibernate must restore a session by accessing the hard drive, and includes some start up patterns, which takes a little longer. Both modes tend to require a password before the information is restored.