Put it in the simplest explanation, "Single malt" whisky is the product of just one distillery and it needs to be distilled using 100% malted barley. For an example, every single drop of whisky in a bottle of The Glenlivet 12 year old can only be made from The Glenlivet distillery.
Then I guess it is not hard to imagine what a "Blended" whisky is. A blended whisky is a mixture of single malt and grain whisky. Usually, whiskies from a range of distilleries might be used. Some well known example of a blended whisky are Johnnie walker Blue label, Chivas 18 year old and Dewars 12 year old.
There is another type of whisky that can be easily and ususlly has been confused with the blended whisky - the name, "Blended malt". The "Blended malt" was previously known as "vatted malts", which means that the product consist the mixture of two or more single malt whiskies. The only difference to a "Blended whisky" that a "Blended malt" can not have a drop of grain whisky, in another word, "Blended malt" would be made from 100% malted barely whiskies. A good example is Johnnie walker Green label.
...Single Malt ................. 1 .................malted barely
..................................................... (wheat, rye, corn)