A Nocturne was original intended to be music inspired by the evening or night-time, but once established the name was given to many evocative works, often relaxed or poetic in nature. Frederic Chopin is one of the most famous composers of Nocturnes, and in total he has 21 pieces of music labelled as Nocturnes. Only 18 Nocturnes were published during Chopin's life. The Nocturne No.20 in C-sharp minor wasn't originally labelled as a Nocturne, but it has all the hallmarks of the form so later publishers called it Nocturne No.20. After a short introduction, the right-hand plays a melodic line while the left accompanies with broken chords. There melody is developed and flows through a number of episodes. The most surprising episode is the middle section which is in 3/4 time whose triplet rhythm in the left-hand seems to give a Spanish quality to the music. The melody returns in an altered format and the piece ends after a series of up and down glissandi.
You can see the Nocturne No.20 below (by scrolling down past the video), or download PDF Sheet Music, MIDI and MP3 files using the links in the left-hand menu. Here is an alternative video which uses a graphical animation to illustrate the music. The complete set of Chopin Nocturnes is highly recommended for the more advanced pianist and is available in a number of editions, such as the Shirmer edition (which has 19 Nocturnes) from Sheet Music Plus (US) or The Music Room (UK); or the Urtext edition (with all 21 Nocturnes but a little more expensive) from Sheet Music Plus or The Music Room.