The harpsichord was the most popular keyboard instrument of the Baroque Period. It looks a lot like a piano but the sound is made differently and the keyboard usually looks different - the black keys are white and the white keys are black.
If you hear harpsichord in a piece of music, then the chances are that it's from the Baroque Period
How does it work?
The harpsichord makes sound by plucking the strings inside when you play on one of the keys.
The plectrum plucks the string and the damper stops the sound when the players lets go of the key. Sometimes there will be more than one string to make the sound louder. There can also be more than one keyboard, like an organ, which also was a way to make the sound louder.
Bartolomeo Christofori was a famous Italian Harpsichord maker. He became frustrated by the fact that the harpsichord could not make soft and loud sound (piano and forte). Around 1700 Christofori introduced his "Grave Cembalo col Piano e Forte" (large harpsichord that can be soft and loud), the instrument that we call the piano today.
Here's a video that explains how the harpsichord works and shows you the inside of two different harpsichords.
Can you hear the difference?
Here are two performances of the same piece by Bach. The first on the harpsichord and second on the piano. Listen to them both. Which do you like better? What do you think Bach would have thought about hearing his piece on the piano?