Molecular forces are forces of attraction between different molecules.
There are four general types of molecular forces:
Type 1 - Ionic forces. These are the strongest types of forces because of the strong attraction between positively charged and negatively charged ions. Crystals of salt are an example of this type of molecular force. These materials have very high melting points.
type 2 - dipole to dipole forces. A dipole exists in a molecule when one part of the molecule has a relative positive charge while another part of the same molecule has a relative negative charge. Hydrogen chloride gas is an example of a dipole molecule. The hydrogen has a relatively positive charge while the chlorine has a relatively negative charge. The molecules in the gas will orient themselves so oppositely charged parts of the molecules are aligned.
type 3 - hydrogen bonding. This is really a type of dipole to dipole attraction because of how common and important it is to life on earth.
type 4 - induced dipoles which are the weakest type of attraction. Because electrons are relatively free to move around within many substances, the electron clouds around the nucleus can temporarily shift. This shift causes a corresponding shift in another adjacent molecule and a small, momentary attraction between the two molecules.