When scientists talk about matter they mean every substance in the universe from the largest planet to the tiniest speck of dust. But matter isn't just solid, all matter can exist in different states, solid, liquid or gas. For example a rock is an example of a solid and the air we breathe is an example of a gas. But it isn't as simple as all that - all substances can change into other states of matter, given the right conditions. This is all because of molecules.
Molecules are tiny particles which make up every solid, liquid or gas.
They can't move about. This is why solids have a definite shape. This means that there is no definite shape. Liquids take the shape of their container. Gases spread out to totally fill the space they are contained in.