Molecules

Any time two atoms join together, they make a molecule. All the stuff around you is made up of molecules. This includes you! You are actually made up of trillions and trillions of different types of molecules. 

Compounds 

When atoms of different types of elements join together, they make molecules called compounds. Water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. This is why it's called H₂O. Water will always have 2 times the number of hydrogen atoms as oxygen atoms. 

Water Molecule showing 1 Oxygen atom and 2 hydrogen atoms