While both liquids and gases are considered fluids, for a liquid, intermolecular forces facilitate the movement of particles. They also have a fixed volume. When a liquid is poured into a container, it takes up a volume less than or equal to that of the container.
On the other hand, gases have separate particles in motion, which collide with each other and scatter; this is why they don’t have a defined volume. This way, they tend to occupy the largest possible volume of the container and are highly expandable.