Diegetic: Perspective is expressed by how the director chooses to emphasise elements in the film. E.g. Star Wars
Intra-Diegetic: Popularly and most commonly used in ‘Found Footage’ horror movies, Intra-diegetic puts the audience in place of a character. A famous example of this is the film ‘Blair Witch Project’
Extra-Diegetic: When a character looks directly at the lens and addresses the audience breaking the 4th wall. A popular contemporary example is Deadpool