I am referring to opera singers. I know that soprano have a lighter voice and higher range up to C6 while mezzo soprano extend to only A5 and have a denser voice but what make the singer a Mezzo with higher notes or a soprano with lower range. For example Maria Callas, I've read a lot of confusing thing about her voice some refer to her as a soprano and other as a mezzo soprano. how can we tell?
Don't let yourself be fazed by reading confusing things about Maria Callas' voice. During the years making her famous as a singer, she covered a ridiculous breadth of soprano fachs, partly at the same time in different productions. In her diva years, her voice deteriorated but she still swept the audience with her interpretation and stage presence.
At any rate, "soprano" falls into several different categories anyway: coloratura soprano, dramatic soprano, lyric soprano, spinto soprano, soubrette. All of those differ in the kind of registers, technique, style and ranges that they use primarily.
Mezzos have a large overlap in total range with most soprano types (the coloratura being notably higher) but tend to work a lot more with the lower parts of the ranges. So someone with a large total range may indeed cover more than one fach in theory though the exercise regimen for being competitive in several fachs may certainly end up daunting.
Mezzo Soprano refers to the range of voice that lies between the contralto and the soprano voices.
A mezzo-soprano or mezzo is a type of classical female singing voice whose vocal range lies between the soprano and the contralto voice types. The mezzo-soprano's vocal range usually extends from the A below middle C to the A two octaves above (i.e. A3–A5 in scientific pitch notation, where middle C = C4). In the lower and upper extremes, some mezzo-sopranos may extend down to the F below middle C (F3) and as high as "high C" (C6).
It's a bit lower than the soprano voice.
Also, here is a video that explains the difference: