The Chopin preludes are a set of 24 short pieces. Chopin published them set in 1839. He'd written most of them before he went on his horrendous trip to Majorca with his lover George Sand. Once he was there, he finished the preludes up.
They're tiny pieces - the longest is only 5 minutes (the well-known Raindrop prelude)! About 8 or 9 of them are under one minute long. But each one manages to convey a very powerful emotion in that small space of time.
The preludes have a very improvised feel - they're like musical sketches. This is because of the way Chopin composed... he just made it up as he went along, trying out different things and writing down what worked.
Some of the preludes sound so free-flowing that publishers sometimes leave the pieces looking like a continuous stream of music on the page. It's as if Chopin just did a "brain-dump" on the page.
Chopin wrote these preludes in all 24 Keys, from A Major to G Minor. He was a dedicated student of Bach, who did the same in the important Well-tempered Klavier (which standardized the western musical keys).
For more detailed information, see this page about the Chopin Preludes.
