The Nocturno is not a genre.
A nocturne (from the French which meant nocturnal, from Latin nocturnus) is usually a musical composition that is inspired by, or evocative of, the night.
The nocturne can be in any time signature, any key, etc... Most nocturnes are for piano solo, but there are orchestral nocturne, too.
The "father" of the nocturne is John Field, although the most famous ones are by Choping. You can listen to all of them on youtube (and see the scores).