Gnat? Mosquito? or fruit fly?

I cant find anything that tells me the differance between a mosquito and gnat? When i was in school and we were learning spanish they said Gnat is english for mosquito? I know there are many families. I just dont want to sound stupid and say hey there goes gnat when its really a mosquito. Recently at my friends house they have these mosquito type bug but it looks like a fly so i assume its those mosquitos people call blackflies? and now i hear fruit flies and gnats are the same? HELP!