Matter exists in three states:
In Solids, the intermolecular distance is very small and the atoms are held together by strong forces. They are in compressible, and they have fixed shape and size.
In Liquids, the intermolecular distance is greater than solids and the intermolecular force is weaker. They are compressible. They have fixed volume but not fixed shape.
In Gases, the intermolecular distance is much larger than that of solids and liquids. So, the intermolecular force is very weak. They are highly compressible. They do not have fixed shape and size. Gases exhibit certain other properties like:
Gases can be expanded by increasing temperature i.e. by heating.
Gases can be compressed by applying pressure.
Gases can intermix with each other to form a homogeneous mixture. This is called diffusion.