What happens to a computers main memory when the computer is turned off or rebooted?

All RAM (Random Access Memory) requires power to work. Once the computer is shut down, the power is lost, and so is all the data stored in the RAM. RAM is designed this way so that it remains incredibly fast. Standby made on Computers saves your current session to RAM, and continues to provide power to the RAM in order for it to continue remembering your session. If such power was lost, so would be your session. Hibernate mode on Computers, however, saves the session to the Hard Drive, which does not require power to remember things. This enables Hibernate to be so much more power efficient than Standby, although Standby is faster to resume.