Diegetic sound are the sounds inside the movieworld, where the characters can hear them, like radios, bands or people talking. Diegetic comes from the greek word diegesis wich means to recount a story. Diegetic is also known as actual or literal sound. Diegetic sounds can be further categorized as source-connected or source-disconnected depending on whether the sound source is visible or implied on the screen. A visible source is shown on the screen, while an implied source is still part of the film world but not shown on the screen.

non-diegetic sounds are the sound that are outside the movieworld, where the characters can't hear them, like background music and narration. Non-diegetic is also known as commentary or nonliteral sound. 

somethimes the line between diegetic and non diegetic fades, for example if it is the thinking proces of a character that you hear. This might seem like a nondiegetic sound but can actually be categorised in a diegetic sound because the character him/her-self can hear it.