All LEDs need some sort of current limiting or they burn out instantly, especially at 12 or 14 volts.
For some specialized LEDs like the Radio Shack one you mentioned, the current limiting or constant current capability is built right into the LED itself, so you can use these units directly with the proper supply voltage.
I assume the 12 volt rating is nominal;all the LEDs I have seen that have a current limiting device built in actually specify a range of voltage over which they will operate from the low end which gives acceptable brightness to something below the maximum.
Using a 12 volt LED makes things easy, but you have very little choice as to colour, brightness, physical size ect.
To use any (non 12V LED), all you need is a resistor in series with the LED to limit the current. You can calculate the value using Ohms Law which was described in this workshop not too long ago or you can use 820 ohms or so for a 14VDC supply which limits the current to about 15ma. The resistor will dissipate just less than 200mw, so a 1/2 watt resistor is just fine.
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Using LED's for some garden railway projects.