The piano is a musical instrument. It is played with two hands, with all 10 fingers at some point during the piece. It is part of the keyboard family.
The strings are hit with a hammer when a key is pressed. The pianos predecessers had other methods like the Harpsichord that plucked the strings alot like a guitar.
The piano by definition is a percussion instrument in that it uses a hammer to produce the note - percussion being any instrument that is hit to make a sound.