Q: What piece is this? A: This Chacone (Z. T680) was based on The Curtain Tune for strings from Purcell's Timon of Athens (Z. 632/20).
Q: What is a Chacone/Ground/Passacaglia A: These are all musical forms in which a repeating bass line (or harmonic pattern) is accompanied by free-composed (non-repeating) material. You can read more about this here: http://en.wikipedia.org/wiki/Ostinato
Q: What instrument is this? A: Booth is performing on a French harpsichord. The instrument was built in 1661, and was recently restored by Booth (who is an instrument maker as well as a performer).