The forces operating between the constituent particles (atoms or molecules), in any form of matter are called intermolecular forces.
In solids, the intermolecular forces are very strong, and the constituent particles are closely packed. That is why; solids are incompressible and have high density.
In liquids, the intermolecular forces are strong enough to keep the particles tied upon to each other but not strong enough to keep them in fixed positions. This is why; liquids can flow and do not have a definite shape.
In gases, the intermolecular forces are negligible (extremely weak), and the constituent particles are free to move. As a result, gases, can occupy any space available to them.