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• Inside the cat's nose, separated by a nasal septum, is a labyrinth of bony platelike projections called the conchae.
• The conchae nearly fill the interior of the nose.
• They are covered by an olfactory mucosa, providing a surface of around 3-6 square inches.
• This size is twice the amount in a human's nose.
• In the mucosa are olfactory cells that detect scents.
• The olfactory cells are only at the top back of the nasal cavity, so the scents only reach the cells if the cat is sniffing (rather than just breathing).
• Below the nasal cavity are also curved cartilage tubes.
• These tubes are called the vomeronasal organ, or Jacobson's organ.
• These tubes connect to small holes behind the upper incisor teeth.
• This arrangement allows the cat to "taste scent" by opening its mouth and allowing the scent to penetrate through the tubes.
• When a cat does this, it looks like it is grimacing, and it also seems to be in a trancelike state.
• This activity is called the "Flehmen response".
• In human's the Jacobson's organ is only rudimentary, so we can't process scent information the same way cats can.

Source: Understanding Cats -- Their History, Nature and Behavior, by Roger Tabor, The Reader's Digest Association, Inc., Pleasantville, NY, 1995, pp. 64-65.