Blended whisky is by far the largest whisky type by volume. More than 90% of all whisky sold worldwide are blends. A typical blended whisky consists of 10-25% malt whisky, and the rests is grain whisky from primarily corn, wheat or rye.
Blended whisky is easy to confuse with blended malt whisky (no grain whisky!) or blended grain whisky (no malt whisky!).
Note that as with all definitions and terms, the details and rules varies between countries. We're mainly talking about the Scotch regulations here.