This is a well known work by Bartók, but what is this exotic thing called a celesta? The celesta was invented in 1886 by Parisian harmonium builder Auguste Mustel. It features a keyboard, and might look a bit like a piano but sounds totally different. It sounds a bit like a glockenspiel, like bells in a clocktower maybe. But it is handled like a piano. What is going on here?
The celesta is indeed very much built like a piano, but instead of hitting strings, the hammers hit metal plates that are tuned to their respective notes. It includes four or five octaves and is written one octave lower than the actual instrument sounds, which is called a transposing instrument.
Although its use in classical music was not that widespread (Tchaikovsky loved it: I suppose it fitted his colourful musicality), the celesta has been used in a surprising number of pop songs, including by the Rolling Stones and The Beatles, but also more recently by funky jazz collective Vulfpack and Icelandic post rockers Sigur Rós. It is also used by Snuffy Walden in the full orchestra version of the West Wing theme song.