[Mbain]

Can anyone define the difference between a linear power supply and a regulated power supply?

[Folsom]

They aren't mutually exclusive.

A linear power supply can be regulated. But so can an SMPS, constant current, whatever. Regulated just means it won't go over the designated voltage or current (whatever it's setup for). Sometimes the regulator device/s also provide some noise bleed off.

[G Georgopoulos]

a regulated ps is designed for constant voltage regardless of a load,an unregulated ps will not keep the voltage constant depending on a load...

[Folsom]

Regulated power supplies typical work by converting AC 115-120v down to something several volts above what they need, so the actual regulation part can only let the desired amount through. It helps prevent AC line small dips in voltage affecting the output that's desired. You need 15v, and have it regulated down from 21v, so you've got 6v of margin before anything is off.