A Piano and an Organ look very similar in their looks but serve totally different purposes as musical instruments. Even though both the Piano and the Organ use a keyboard primarily, the way they are used to create music is very different. It will not be only apt to say that apart from the fact that both are keyboard instruments; there is nothing that is similar in both these instruments. In fact there are more differences between the two than there are similarities.
The primary difference between an Organ and a Piano is that, while a Piano is grouped under the percussion family, an organ is more an instrument of the woodwind or brass family. Both the instruments produce sound when a key is struck. In a piano, they keys are mechanically attached to a hammer which strikes metal string with high tension and produces the sound. On the other hand, the key in an organ is connected to an electrical circuit. Whenever the key is depressed, it completes the circuit and sounds are produced. The ease with which either of the instruments can be played determines the major difference between the two. While some feel it is easier to play the piano, some others may feel the organ is comparatively easier to play.
Any instrument that produces sound by striking, plucking or strumming is called a percussion instrument. Since the piano is played by striking the keys to produce sound, it is grouped under the percussion instrument category. Each of the keys of the piano is tuned to a pitch and it is possible to produce a melody or tune by striking the keys in a continuous manner. But it is difficult to sustain the sound for long and to reproduce the sound; the pianist will have to strike the key again to again.
The organ produces sound basically because of the passage of air through pipes (Pipe Organ). Whenever a key in an organ is pressed, the air within the metal pipe is mechanically compressed and resonates within the pipes thus generating a sound. This is very similar to the way sounds are produced in woodwind or brass instruments and hence it is grouped with these instruments. The advantage of the organ over the piano is that the sound produced has a sustaining effect and the organ player can retain this sound for longer periods of time.
An Organ has various advantages over the Piano. While a Piano sounds just like a Piano, an organ can produce the sounds of other reed and woodwind instruments. This is possible due to the presence of multilevel keyboards operated by both hands and feet. The sound produced in a piano cannot be sustained for long while the sounds by an organ have a longer sustaining effect. To sustain a note, the keys in a piano have to be struck again and again while it is possible to sustain or prolong a note in an organ by simply keeping the key pressed for a longer duration. An organ is more versatile than a piano. It can produce sounds of extra accompaniments by striking the musical keys and can keep the sound playing continuously even without pressing any key.
The functions of the Piano and the Organ are very different when used as an accompanying instrument in a choir. Being a percussion instrument, the initial power of the piano lies in the first keyboard strike, often making it a leading instrument in a choir. Since the piano can produce rhythms and melodies faster, it is most often used as the introduction piece to a song. The Organ on the other hand can only be used as a follower as it has long and sharp notes which are most often used to fill in the gaps left by the percussion instruments. Also, the Piano is most often used to play only contemporary music as against the organ which can play more modern sounds along with back-up reverberations. While a Pianist will have to learn complex chords and fingerings, an organ player will have to learn the dynamics of playing the bass notes with the foot keyboard while controlling the various volume pedals efficiently.