What is the difference between a sitar and a tanpura?

Both the sitar and tanpura are string music instruments.  The sitar has frets and is used for composing and playing complete melodies.  Sitar players are like guitar players who perform solo or as part of an ensemble band.  The tanpura on the other hand does not have any frets and is only an accompanying instrument.  Its strings provide a continued background rhythm to a music performance.   While the tanpura alone cannot be used for composing a melody, a music performance is incomplete without a tanpura.