A solid is a state of matter with a fixed or set volume and shape. The atoms and molecules within solids are packed so tightly together that they can't move, only vibrate and turn in place.
A liquid has a fixed volume, but takes the shape of whatever container its in. This is because the molecules in a liquid have more energy than in a solid, allowing them to move and slide freely past each other, allowing the shape to change.
A gas, meanwhile, has no fixed volume or fixed shape. Gas molecules have more energy than solid or liquid molecules, and gas molecules move freely inside whatever space they reside in.
Plasma is the fourth state of matter, and plasma molecules have more energy than the molecules in any other sate of matter. Examples of plasma are the sun and lightning.