Voltage regulator vs. voltage reference
A voltage regulator is designed to take a variable voltage in (say, 2-5v), and output a constant voltage (say, 3.3v).  Now, voltage regulators are typically used to power a circuit, which means they will have a current output of a few hundred mA or more, generally speaking.  In order to keep cost, size, etc down, the output tolerance on voltage regulators are (again, generally) a few 10s or 100s of mV.

For example, the RG71055 voltage regulator has a minimum output voltage of 5.2v, and a maximum of 5.8v, with a target output voltage of 5.5v, and can source 30mA.  That's about a 5% voltage tolerance, assuming I number crunched correctly.

On the flip side, a voltage reference is designed to take a variable voltage, and deliver EXACTLY the rated output voltage.  For example, the LT1790 can supply 5v with a tolerance of 0.1%, which is a 50x improvement over the RG71055.  However, the LT1790 can only source 5mA max, which is 6x less than the RG71055.  A voltage reference is used when you need to know that this line is exactly a certain voltage (in other words, really tight tolerances).  On Digikey, you can get a voltage reference with 0.01% tolerance.  With voltage regulators, you'd be lucky to get one with a 1% tolerance.