Fluids differ from solids in that they flow in response to an applied force. They tend to form faceted crystals if grown under the right conditions. Solids have a nonzero shear modulus. This shear modulus is a material property that tells us how a substance responds to shear forces, which change the shape of the substance without changing the volume. Finally solids have a fixed shape at low pressure and are usually hard to compress.
Liquids have a fixed volume at low pressure and like the solids they are usually hard to compress.
Gases on the other hand expand to occupy the available volume and are highly compressible. A solid tries to resist static shear distortions, while a fluid does not. Conversely, some liquids are highly viscous and change shape only slowly. Glasses are liquids that increase their viscosity enormously as they cool, but can behave like brittle solids under high stress. Obviously, rigid solids and freely flowing fluids are just extreme cases.