Highlighting--bleaching selected strands of hair, often concentrated around the face -- is very popular, and with good reason: it adds visual interest to hair without causing nearly as much damage as allover bleaching, and unless you've gone drastically lighter, the roots won't be as noticeable. However, if your hair is very dark, highlighting may only give you reddish or light brown tones -- it's difficult to convince it to go blonde. Lowlighting is the opposite process of highlighting -- small amounts of hair are darkened.
There are three ways to highlight hair:
Foiling, done only in salons, involves applying bleach to many fine strands of hair and individually wrapping them in foil. This process is the easiest to control and can be done closest to the roots; it usually will get you the best results.
Hair painting, which can be done at home or by a colorist, is simply applying bleach randomly to the hair with a small brush.
The third -- and oldest -- method doesn't produce very good results and is out of favor at salons, though it's still available in home kits. You place a plastic cap with small holes on your head, pull strands of hair through the holes with a small hook and then bleach them.
There are benefits to having these processes done by a professional: A colorist can give you a few different levels of highlights or lowlights (or both at once) for a more subtle look. (Sometimes they'll use a toner after highlighting to get just the right shade.) And of course, a pro can see the top and back of your head better than you can.