A gas is a substance with no definite volume and no definite shape. Solids and liquids have volumes that do not change easily.
A solid has a definite shape and volume. There are two kinds of solids—crystalline and amorphous.
Liquids can occupy whatever shape their container has, so they do not have a fixed shape. Because the particles in liquids are very close together (barely further apart than in solids) liquids do not easily compress, so their volume is fixed.