  We present a two-channel model to describe the quantum state of two atoms
with finite-range interaction near a Feshbach resonance. This model provides a
simple picture to analytically derive the wave function and the binding energy
of the molecular bound state. The results agree excellently with the
measurements and multichannel calculations. For small binding energies, the
system enters a threshold regime in which the Feshbach molecules are identical
to long range atom pairs in single channel. According to their threshold
behavior, we find Feshbach resonances can be classified into two types.
