Power saving states

C state refers to core state which power to the core parts are turned off to save power. There are different C state level with progressive longer exit latency. 

Package C state applies the techniques beyond the core to other components in the package like cache, integrate PCIe controller etc

P state refers to power state which frequency and voltage scaling is used to save power on the core or package level 

T state is to duty recycle core (shutdown the core). This is not so much to save power but to throttle to reduce thermal and power issues. 

S state refer to sleep state which drive the poster to near zero. This is usually employed in workstation but can also used for server. 

G state refers to global state which is similar to S state but apply to the whole platform. 

D state refers to device state which component like PCI card or drive is powered down to save power.