In order to study tissue under a microscope, that tissue must be very thin.  Most of the tissue to be studied is cut below 15 microns (um).  A micron is 1/1000 of a millimeter.  Actually 15 microns in histology is considered thick.  Routine paraffin and cryosections are cut at 5 microns.  This is typically 2-3 cell layers thick.  After it is cut, the sections are placed on top of a glass microscope slide.  The slide usually has some kind of adhesive to keep the tissue from falling off.  The slide is then dried in a 60C oven or left overnight.  This ensures no water will be on the slide or tissue.  It is then dipped in a series of chemicals to stain the tissue and make it permanent for later viewing under a microscope. It is this final product that is considered a histology slide.

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