Acoustic piano

The term "acoustic" piano means the sound is produced by 'natural' means without any electronics involved.  There are two basic types of acoustic piano, the Grand and the Upright.

Grand Piano

The frame and strings in a grand piano are placed horizontally, with the strings extending away from the keyboard. This makes the grand piano an instrument at home in a large room with high ceilings for proper resonance.

 The "concert grand" is just what it says - suitable only for a concert hall.  Its size can be between about 2.2 m to 3 m long.  A "parlor grand" is about 1.7 m to 2.2 m and the smaller "baby grand" could be suitable for a large domestic lounge, being shorter than it is wide.

All else being equal, longer pianos with longer strings have better sound on the lower strings.  The longer strings on a concert grand piano can vibrate more freely than the shorter, thicker strings on a baby grand, which means that a concert grand's strings will have truer overtones.

Upright Piano

Upright pianos, sometimes known as vertical pianos, are more compact because the frame and strings are placed vertically.   It's more difficult to produce a sensitive piano action when the hammers move horizontally, as the hammer return relies on springs which may wear and tear, whereas the grand piano hammers return by gravity.

However, a well-regulated upright piano will probably play smoother than a grand piano that has not been regulated for years.   The finest upright pianos can now come close to the quality of someme grand pianos of the same size in tone and responsiveness.

One significant advantage that the grand piano has over the upright is that all grand pianos have a special repetition lever in the playing action that is absent in all verticals. This lever, a separate one for every key, catches the hammer close to the strings when the keys are played repeatedly and quickly. Thus, the pianist can play repeated notes, staccato, and trills with much more speed and control than is possible on a vertical piano.

Electric piano