Matter commonly exists in three phases:
A solid retains its own shape and volume and does not "flow."
A liquid retains its own volume, but takes the shape of its container.
A gas takes the size and shape of its container.
A fluid is either a solid or a gas.
Plasma is a special state of a very hot chemical element whose atoms are completely stripped of their electrons. Plasma is recognized as a separate state because it has properties that distinguish it even from gas.