1) dispersion or London forces :
dispersion forces are weak forces that result from temporary shifts in the density of electrons in electron clouds .
When two non-polar molecules are in close contact , especially when they collide , the electron cloud of the one molecule repels the one of the other molecule .When temporary dipoles are close together , a weak dispersion force exists between oppositely charged regions of the dipole .
2) Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule.
3)Hydrogen bonds :
One type of diploe-dipole attraction is called hydrogen bond . it occurs between molecules containing a hydrogen atom bonded to a highly electronegative atom with at least one electron pair.

A. Terminology
We can breakup the word intermolecular into two parts , inter and molecular . INTER means between and MOLECULAR deals something with molecules thus intermolecular is a force between molecules.

B. Definition
The strength of attractive forces between molecules are called  intermolecular forces .They are attractions felt between atoms and molecules .
C. Importance
They play a very important role in chemistry .It can determine how quickly a reaction can occur and it can determine whether a material will be a liquid , a solid , or a gas at a given tempreture.