There are two basic types of scotch: single malt scotch whisky and blended scotch whisky.
Only water and barley are used to make scotch whisky. All scotch whisky must be aged in oak barrels for a minimum of three years.
A combination of one or more single malts, blended together with one or more single grain whiskies to make blended scotch whisky.
Johnnie Walker Black Label, a great blended scotch and a nice introduction to the world of scotch:
Why is single malt generally regarded to be "better" than blended scotch?