Basically Scotch falls into two categories, blends and single malts. Scotch whisky uses the blending process to enhance the taste of the whisky, whereas American and Canadian blenders use blending to hide the deficiencies of the individual whiskies. Blending in itself is a unique art form. Some malts may be what are called vatted malts which are blends of several single malts. But basically the single malts represent a single, pure distilled Scotch whisky.
Single malts fall into several sub-categories.
These are Highland malts, Islay malts, Lowland malts, and Speyside malts. Each has its own special characteristics. I suggest checking out the Scotch Whisky Association's web site for more complete information on the differences between the various sub-categories. Aging is also important to Scotch. This includes the whiskies used for blends.