Interstellar gas occupies the space between stars. It is a very thin gas with, on the average, one atom in a volume the size of your thumb. The interstellar medium is is made up of interstellar gas and dust. The gas comes in patches that astronomers call interstellar clouds. Clouds can be made mostly out of ions, atoms, or molecules.
Stars form out of the densest clouds which astronomers call molecular clouds.
Most of what we do in radio astronomy is the study of the interstellar medium. Molecular clouds are patches of the interstellar medium that are so dense that molecules can form. In molecular clouds, the gas gets dense enough that stars can form out of the gas. Molecules form when two or more atoms combine together. Most of the molecules in space can only exist in molecular clouds where the interstellar medium is cold and dense.