Relief and safety valves prevent equipment damage by relieving accidental over-pressurisation of fluid systems. The main difference between a relief valve and a safety valve is the extent of opening at the set-point pressure.
A relief valve gradually opens as the inlet pressure increases above the set-point. A relief valve opens only as necessary to relieve the over-pressure condition.
A safety valve (shown on image) rapidly pops fully open as soon as the pressure setting is reached and will stay fully open until the pressure drops below a reset pressure. The reset pressure is lower than the actuating pressure set-point. The difference between the actuating pressure set-point and the pressure at which the safety valve resets is called blowdown.