1) Heterogeneous mixture is not uniform in composition.
For example, sand and small pebbles, proportions of its components (in this mixture particles of different colors and size) vary throughout the sample.
2) Homogeneous mixture (solution if it is liquid) has the same proporties of its components throughout any given sample.
For example, if we have mixture of water and alcohol ethanol, we can separate water and ethanol with distillation (process of heating and cooling). Ethanol and water have different boiling points (physical property).
3) Pure substance is made of only one type of atom or only one type of molecule, it has definite and constant composition with distinct chemical properties.