The basic difference is in production. Single malt scotches are strictly made from malted barley, whereas blended scotch whiskies come from a variety of grains such as wheat, corn and barley. Single malt whiskies are also sourced from a single producer at a single site, whereas blended scotch is made by mixing different distillations from multiple producers.
Single malts may be a blend of malts from various ages, but they all come from the same distiller. They variety of ages can be used to heighten the character and sophistication of the blend, but at the same time the character of the product must be retained and batches must be kept as consistent as possible. The age that appears on the bottle is the youngest age included in that product, although the single malt may include other whiskies from the same distiller that have been aged for longer. (If you want a malt whisky that comes from a single distillation from a single producer, you’re looking for a single single malt. And if you want a single distillation from a single producer from a single barrel you’ll need a single singe single malt, aka direct bottling.)
Blended scotches are a more cost effective product, since grain whisky is cheaper to produce. Whisky made from unmalted cereal such as wheat or maize tends to have a harsher flavor than whisky made from malted grains. The challenge in produce a consistent blended scotch arises when a base component of the blended whisky becomes unavailable, either because a distillery has closed or no longer wishes to offer their product to blenders.
Tip: Experts say adding a couple of drops of water to your scotch will help bring out the flavor, or take a sip of coffee or a bite of dark chocolate to better appreciate the subtle tones of the scotch.