States of matter (phases) are fundamental manifestations of matter, which differ abruptly in the mobility of their atoms and molecules as well as in the strength of interactions between them.
Gases have particles that are freely movable at a great distance from each other and continuously fill the available space. Gases are compressible, the volume of ideal gases is inversely proportional to pressure.
Solids are substances that have a solid state at standard conditions. Their particles can be moved only minimally, deformed by forces(elastic or plastic).
Liquids are substances that have a liquid state at standard conditions. Such materials offer virtually no resistance to shape change, but quite a large resistance to volume change.