Fluorescence Microscopy & Imaging Center
A confocal microscope significantly improves resolution over a conventional microscope by removing out-of-focus blur from the image as it is formed. It does this removal optically, but requires scanning of the sample focal plane in order to build up a complete 2-dimensional image. For more explanation of confocal microscopy, try the following links:
The only requirement for a microscope to be "confocal" is that, in the image plane, there is an aperture-- or a grid of apertures placed apart from each other-- that serves to remove the out-of-focus light coming from the sample. In practice, the geometry of this aperture (or apertures) is matched by a corresponding illumination pattern at the sample focal plane-- which serves to increase resolution further-- and the illumination pattern is scanned across the sample in order to build a complete 2-dimensional image.
Confocal microscopes, as they are currently implemented, are a type of light microscope. They are used mostly (or entirely?) either as fluorescence microscopes in the biological sciences, or as reflection microscopes in the material sciences.
The commercialization of the confocal microscope since the 1980s represents a major revolution in light microscopy. It is important to realize that it is only one of several ways-- multiphoton excitation, deconvolution(http://www.niehs.nih.gov/research/atniehs/core/fmic/software.cfm#deconvolution), TIRF, etc. being other ways-- to improve resolution in light microscopy. Although each of these techniques has its own strengths for particular applications, confocal microscopy provides the lowest price/performance on average; thus it has become the most popular technique for 3D light microscopy.