A smell is simply our body's way of identifying certain chemicals that occur in the fluid medium (gas or liquid) that we live within and informing us about the particular chemicals that it detects in this medium.

For humans, smell detects chemicals in the air. For fish, smell detects chemicals in the water. Both air and water are fluid mediums that carry with them many other chemicals. Smell is a way of sampling this medium and providing us information about the environment we live in. The sense of smell is also known as olfaction, the olfactory sense, or a chemosensory mechanism. I don't know exactly how the sense of smell works, but it usually involves a chemosensory organ with many nerve endings that are highly sensitive to different types of chemicals. In our body, this organ would be our nose. When we inhale through our nose, we draw air into our nasal cavity along with a multitude of different chemicals that are mixed with this air. The air and these other chemicals contact the millions of nerve endings in our nasal cavity.

Some of these chemicals are odorless because they don't trigger any responses of the nerve endings, while other chemicals cause certain nerve endings to fire signals towards the brain. The brain receives these signals, interacts with your memory center, and identifies what chemical you smell or alternatively tells you if the smell that you sense is unfamiliar. Quite a bit of research has shown that the brain is very adept at memorizing smells and has a strong ability to recall or identify specific smells many years after it first detected them.

I don't know exactly how the process of sensory nerve signaling for smells works. However, I think that what happens is the following: our nose (and tongue) is full of many different nerve cells that react to many different types of chemicals. Each nerve cell has a membrane that binds to the specific group of chemicals that it evolved to detect. When that specific chemical is present, it binds to the nerve cells membrane and causes a change in the permeability of the membrane, allowing the transport of ions (charged particles) across the membrane. This movement of ions across the nerves membrane creates an electrical charge that causes a current to flow down the length of the nerve all the way to the brain. This current is essentially how the brain transmits information.

When the brain receives this current, it can identify what type of nerve cell the current originated from and therefore knows what group of chemicals the nose detected. I think this is how we smell.

The tongue is also a chemosensory organ involved in taste, which is called the gustatory sense. The tongue and taste operate in a very similar manner as the nose and smell by detecting chemicals with many sensitive nerve endings (located on the taste buds of our tongue and in the nasal cavity of our nose) and then communicating this to our brain. But the purpose of the gustatory sense is not to detect chemicals in the fluid medium of our surrounding environment, as in the olfactory sense, but rather to detect chemicals that we ingest into our body through our mouth (i.e., in our food and drink). However, this is a fine line to draw and may only work as a distinction between smell and taste for humans and a few other mammals.

Much research has indicated that the sense of smell and taste (the olfactory and gustatory senses) work very closely with one another and are dependent upon one another to accurately sense our chemical environment. Some studies have suggested that our sense of taste is heavily impaired if our sense of smell is damaged, indicating that much of what we think we taste is actually smell. This is why food tastes bland when you have a cold: because your sense of smell is dulled. Also, many other animals use their sense of taste to smell the solid parts of their environment in order to distinguish food from foe and edible from inedible. Some animals even have sensitive taste buds on body parts such as their feet so that they can identify chemicals in their environment everywhere they step.

The only consistent distinction between smell and taste that I can think of is that smell is used to sense chemicals contained within the fluid medium of one's environment (gas or liquid), whereas taste is used to sense chemicals upon specific surfaces in ones environment. In other words, smell gives you a sense for what chemicals are all around you (e.g., in the air or water), whereas taste gives you a sense for what chemicals are at particular locations or on specific items (e.g., food, surfaces, etc.) that you can sample selectively.

There are proteins on the surface of cells in our nose; called receptors that recognize and bind certain molecules in the air and through a variety of mechanisms relay the information to our brain allowing us to identify the smell. The cells with these odor receptors are found in an upper region of our nose called the olfactory epithelium, which is covered by a mucus layer. There are hundreds of different odor receptors in our nose and each one can recognize certain odor molecules.

Altogether, we have about 40 million odor receptors (many which are identical and recognize the same molecule). Interestingly, a German shepherd dog has about 2 billion odor receptors. When the nasal receptors recognize a molecule, a signal is sent through the olfactory nerve to the olfactory bulb. The olfactory bulb is just above the nasal cavity underneath the front of your brain. Signals are sent from the olfactory bulb to other parts of the brain to be interpreted as a smell you may recognize. These other areas of the brain include areas responsible for emotional behavior and memory. This is why smells often remind you of certain memories.

Smell is the sense by which cells on the inside of your nose and on your tongue (mostly in your nose)have chemical compounds on their surfaces that react to chemicals in the air, and this sends a cascade of reactions back to nerve signals that coordinate your body (most, but not all, of this coordination takes place in your brain). Depending on which cells detected chemicals, you know which chemicals they are, which delineates different smells.

A smell is a percept, just like a visual percept (seeing) or auditory percept (hearing). The scientific study of smell is called olfaction.

When you breathe, air goes to the olfactory epithelium located below the back of your eyes. This in return triggers an electrochemical signal that is sent to the olfactory bulb of your brain. Quite interestingly, the actual motor process of sniffing is associated with different activity in your brain whether an odor is present or not.