"Movies" vs. "Cinema" vs. "Theater" -- what's the difference?

Movies is slang for a motion picture. Film is the medium on which motion pictures are fixed. Cinema is from the French cinématographe which comes in part from the greek kinema, meaning movement. So cinema is really just another word meaning moving picture. It also has come to mean more generally the process of film-making and also the building where films are shown. Theater is similar to cinema, in that it can mean the building, or more generally the industry of live performance (i.e. plays, musicals, etc).

Film, movies, and pictures are used interchangeably:

I saw a film. I saw a movie. I saw a picture.

In context, the theater is the building where movies are shown, but usually people would specify the movie theater to avoid confusion with the live theater.

"Movies" and it's short-lived companion "talkies" describe in a very simplistic way, what it going on on the screen. Things are moving: movies. People are talking: talkies.

Movies and pictures can be used interchangeably:

I'm going to the movies/pictures.

But pictures is mainly a UK expression.

I don't know about US, but mostly in UK we go to the cinema to watch a film on the big screen. Sometimes you still hear pictures, or flicks, but they're getting a bit dated. I don't think many people would say they went to the theatre without adding something to indicate this was to see a film, since by default it would be assumed the entertainment was something with live performers on stage.

I think we're less likely than Americans to say, for example, Most Friday nights I go to the movies. In the cause of lingustic globalisation we do go to see a movie, but not so often as we watch a film (but of course, you can also do that at home - in your home cinema if you've got one).

Movie/movies is an American word. Theater is the American spelling of theatre. From my experiences as a British person, I can tell you about this.

Americans talk about watching “movies”, going to “the movies”, or watching them in a “movie theater” — which they also abbreviate as just a “theater”.

British people talk about watching “films” or going to “the cinema”. For British people, a theatre (which is the British spelling as opposed to the American spelling) is somewhere to watch a play on stage. It can include musicals. It has this meaning only.

However, in very recent years, the word movie has slowly started to be used here in the UK as well. It is not widespread, but is used by some British people, mainly radio and TV presenters.

"The movies" is chiefly American. "The cinema" is chiefly British. "Theater/theatre" is said of places where plays are shown, however if you modify it to "movie theater" then you have a theater where films are shown. "Movie house" can also be used.