ACPI Sleep states

S0	The system is powered and all systems are operational – lowest level of power saving.
S1	(Power On Suspend, Pos) The system is powered, all system context is preserved.
S2	The system is powered on, but CPU and cache context are not preserved.
S3	(Suspend to RAM, STR) The system is powered, all system context is lost, but system memory retained.
S4	(Suspend to disk, Hibernate) System is powered off, all context is saved by OS to disk, power can be removed.
S5	(Soft-Off)Same as S4, but on context saved by the OS.