Curious - What is the moon shaped key all about

This button puts the computer into sleep mode.

FROM WIKIPEDIA-

Sleep mode can go by many different names, including Stand By (Microsoft Windows 95-Server 2003), Sleep (Mac OS 8)-(Mac OS X) / (Windows Vista,Windows Server 2008), and Suspend (Linux). When placed in this Sleep mode, aside from the RAM which is required to restore the machine's state, the computer attempts to cut power to all unneeded parts of the machine. Because of the large power savings, most laptops automatically enter this mode when the computer is running on batteries and the lid is closed.
Though Sleep and Hibernate are generally thought as two separate functions, modern Macintoshes, Windows Vista (termed Hybrid Sleep), and some specially configured Linux machines also feature a variation of Sleep mode that incorporates the strengths of Hibernate. The RAM is copied to the hard drive before sleep mode is initiated, preserving the RAM contents in the event of a power loss.
ACPI is the current standard for power management, superseding APM and providing the backbone for sleep and hibernation on modern computers. Sleep mode corresponds to ACPI mode S3. When a non-ACPI device is plugged in, Windows will sometimes disable stand-by functionality for the whole operating system. Without ACPI functionality, as seen on older hardware, sleep mode is usually restricted to turning off the monitor and spinning down the hard drive.

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