The piano took the harpsichord's place as the dominant keyboard instrument of the Romantic era. It was originally called pianoforte because, unlike the harpsichord, the player could control its loudness and softness with the amount of pressure applied to the keyboard. The mechanism involves small padded hammers that strike the strings when a key is struck. The first pianos began being built as early as the 1720s, and J.S. Bach is known to have tried playing one.
Since the Romantic era, the piano has been wildly popular as a solo instrument. But sometimes composers write orchestral parts for the instrument, too. Two good examples are in Stravinsky's Petrouchka and Aaron Copland's Appalachian Spring.