Trying to decide what lens you want to buy can be confusing. Adding to that confusion is the array of mystery letters in a lens's description. Each of these letters is important in understanding the specifications of the lens. Some of the letters are brand specific but some are fairly standard across brands.
Extra-low Dispersion - This means that the lens has very little color shift when focusing. Colors don't always focus at the same point (think 3D glasses). ED lenses are designed to minimize this problem and reduce the color fringes sometimes seen around photographic subjects. One of the most common fringes is purple (combination of out of focus red and blue) and this purple fringe degrades image sharpness.
Internal Focus - This means that instead of the lens rotating and telescoping as it focuses, the focusing action is done inside the lens. This leads to a more compact and quick lens.
Image Stabilization - This is the technology used to allow for hand-held images as slow shutter speeds. It basically allows the internal workings of the lens/camera to match the motion of your body so as to counteract them. Some brands place IS technology in their camera bodies but other brands place the IS technology only in certain lenses.
Macro - Macro lenses are lenses where the optics allow for high magnification of subjects. "True" macro lenses record the image at the same size as it appears in real life. This is a 1:1 macro. Not all macro lenses are actually "true" macros. Some produce much less magnification than 1:1.
Ultrasonic Motor - This is a special type of motor within a modern autofocus lens that allows for extremely quick focus shifts with very little sound.