There are many types of organs.
A pipe organ forces air through many large and small pipes, each of which produces a unique pitch and timbre. This is the largest, oldest, most majestic, most expensive, most complicated, and arguably most expressive type of organ.
A reed organ may be the size of an upright piano, or much more portable, and forces air through metal reeds much like a harmonica. It sounds similar to an oboe or an accordion.
A Hammond organ has rotating tone wheels which produce various overtones which when mixed together in various levels produce a wide variety of sounds which are projected through rotating Leslie speakers. It has a very pure tone like a flute, but when distortion is applied may sound very "crunchy" like an electric guitar.
A digital organ is a sampler; it stores the sounds of many pipe organs and faithfully reproduces them with speakers on an appropriately grand machine with many registers and stops.