What's the difference between Constant Voltage and Constant Current drivers?

With Constant Voltage the LED module only requires a fixed voltage, often 12VDC or 24VDC. The current is usually regulated by either resistors that have been wired in series with the LEDs or by an onboard built-in regulator driver that the LED module may have. This arrangement is often used in signage where it is not known prior to the installation how many LED loads there will be and in linear architectural designs where there are a number of LED modules connected together. Constant voltage loads may be added in parallel across the output of the driver until the maximum output current is reached.
 

With Constant Current the reverse is true, the current is fixed by the driver while the voltage will vary depending on the load. The more LEDs are attached, the higher the output voltage will be, up to a maximum set by the driver. This is usually used when the load/number of LED's is known and you want optimal efficiency, as in a recessed downlight or track fixture.