It is said to have been invented by Bartolomeo Cristofori, who originally called it a "gravicembalo col piano e forte" -- a large harpsichord with soft and loud. "Pianoforte" stuck as the name for the instrument.
Early pianos had wooden frames, two strings per note, and deerskin-covered hammers. The development of the modern piano owes much to the collaboration between Beethoven and the English firm of Broadwood: as Beethoven grew progressively more deaf, the instruments that Broadwood sent him grew progressively larger, louder, and more robustly constructed -- iron frames, three strings per note, the modern felt-covered hammer.
Much great music has been written for the piano...
A person who plays a piano is known as a pianist.