Pressure relief valves and pressure regulators - what's the difference?

What is the difference between a pressure reducing valve and a pressure relief valve? pressure-reducing/regulating/ relief/release - There are so many different options available!

I read this article on Fluid Controls that did a really good job of explaining pressure regulators, but I am a bit lost on the difference between regulators and relief valves.
A relief valve sets the upper pressure limit, while a regulator maintains a constant pressure. You could also say that the relief valve acts as a shunt to drain, while the regulator acts as a variable resistor in series.