Lens flare occurs when sunlight strikes the front glass and causes light spots, colored hexagonal reflections, or light streaks to appear on the image. Flare can also cause a general washed-out or hazy look. You can actually see this in the viewfinder as you shoot, as well as afterward in the finished image. Occasionally it can be an artistic effect, but most times itís an annoyance.
A lens hood works most of the time for blocking the sun from the lens glass, but if the sun is barely outside the picture area, you may need to shade your lens with your hand or a hat to obstruct the sun. This is yet another worthwhile motivation to use a tripod whenever possible, since itís not easy to hold the camera steady with one hand while blocking out the sunlight with the other. Double-check the results on the back of the camera.
Note: If you can see even a portion of the sun in the viewfinder as you are composing your picture, then itís impossible to eliminate the flare completely. Some unsightly spots can be cloned out in post-processing, though not all - another good reason to pay attention to flare during the shooting process. Avoid using more than one filter at a time, since extra glass can mean extra flare.
More information... This article is adapted from the following book by Jim Miotke and Kerry Drager: The BetterPhoto Guide to Photographing Light