"LED monitor" is a misnomer; both LCD monitors and LED monitors use a liquid crystal (LC) matrix to display the picture. (True LED displays do exist, but they're huge and only look good at a distance, and are used mainly at sporting events and such.)
The difference between what's called an LCD monitor and a LED monitor is the backlight that they use. The liquid crystal matrix itself doesn't emit any light, and light is what you need to see the picture. LCD monitors use any number of backlight mechanisms (usually fluorescent lights), but LED monitors use only LEDs for backlighting. LEDs create much more light for much less power, plus they're nontoxic, so getting a LED monitor both gives you a brighter or darker image and is more environmentally friendly.
The difference is ONLY in the backlight; in theory, if the same liquid crystal matrix is used in both kinds of monitors, they'll both have the same refresh rate and the same color capabilities.