A substance is a sample of a specific type of matter, such as:
-water
-dirt
-
A mixture is a combination of two or more contrasting substances, some mixtures fully combine to create a new substance and some mixtures simply create combinations such as:
-A mixture of dirt + water creates mud which is an example of a new substance
-A mixture of cheese + ham + bread creates a sandwich which is an example of a combination
Hope this helps :)