Dictionary Definitions: 

Intermolecular – “Existing or taking place between molecules.”

Intramolecular – “Existing or taking place within a molecule.”

Selected from OxfordDictionaries.com

Functional Definition:

While the dictionary definitions are very straight forward you still might get inter- and intra- confused because they sound very similar. Inter- refers to between where intra- refers to within. Remembering the prefix meanings may make distinguishing between molecular forces and interactions a bit easier. Examples of intermolecular forces are London Dispersion forces and hydrogen bonding. Intramolecular forces are what hold molecules together such as ionic and covalent bonds.