The document defines the three states of matter as solids, liquids, and gases. Solids have a fixed shape and volume, liquids can flow and change shape but maintain their volume, and gases take the shape of their container and flow easily. Matter can change between these three states through processes like melting, freezing, boiling, and condensation, but the properties of the matter remain the same. The document provides examples like ice, water, and air to illustrate the different states of matter.