Confocal microscopy is a kind of microscopy which gets around a limitation of standard microscopy. It is an important tool in the study of biology.
In standard microscopy, light scattered by regions of the sample that is out of focus interferes with the image of the focal plane. In confocal microcopy, the light source is replaced by a laser. The laser is focused to the same focal plane as the optics of the microscope. The laser is scanned accross the focal plane, and the image is integrated electronically. This results in an image where there is little interferance from out-of-focus regions.
Confocal microscopy is usually used in immunofluorescence microscopy, where one uses fluorescent tags to mark parts of the sample you wish to visualize.
Confocal microscopy is also being replaced by, or being in used with computer de-convolution of the image to mathematically remove the out-of-focus interferance.