Solid. Solid objects have neatly stacked and compacted molecules that have a formed shape. A Rubix cube is a good example for showing how molecules are compacted. Unless great force external force is applied, molecules move together and don't change their shape/form.
Liquid. Liquid molecules such as in this water are slightly further apart than that of a solid and are free moving inside of the liquid. They take shape of the container they are put in and movement transfers between molecules very quickly, creating waves.
Plasma has a neutral charge, with an even number of negative and positive ions. The ions (previously electrons) float around unattached to the protons and neutrons. Plasma does not have a difinite shape or volume ( unless put in a container).
Gas. Gas molecules are spread far apart from each other, and expand and compress to fill their container. They have lowered density than liquids and solids and have no fixed volume or shape.