LED bars are strips of LEDs that you can bolt underneath your counter to create continuous light. Many are sold as rulers, or a single foot of LED light that you can space as needed. Cabinet ropes are a cheaper version; they are flexible ropes dotted with LED lighting. The light quality is poorer, but the ropes are more flexible than bars and can be easily installed under awkward cabinet arrangements.
LED stands for light-emitting diode, a method of producing light using electrical resistance. The diodes are doctored with a variety of elements that causes some of the current to be displaced into the visible electromagnetic spectrum. The result is highly efficient light "bulbs" that last far longer than any other type of lighting. Multiple residential applications exist, and when it comes to under-counter lighting, a number of LED options are available.
The alternative to bars is spot lighting, which is sometimes called puck lighting because of the disc-like shape of the LED panel. These lights may not be marketed specifically for under-cabinet uses, but they can be handy if there is only a single corner or spot that you want to illuminate along your counter. Spot lighting can save money through such customizable placement, and is often simple to operate with basic button designs that turn on with a push.
LED light can be harsher than other types of light, even fluorescents. The bright white LED color is typical for bar lights and other options, but you may want to visit a store and see if you can actually see a variety of LED color options before making your choice. When possible, opt for a warm-white or soft-white option that is color-corrected through filters or different diode elements.
Some LED lights are dimmable. If you want to dim your under-counter lights choose these options. Dimmable LEDS will be more expensive, however, and require more complex electrical work, which can open up new ways for the diodes to fail or malfunction. The basic on/off versions are less costly, easier to install and more straightforward to diagnose if something goes wrong.