Olfaction, or the sense of smell, is the first sense that is activated when we're born. It allows us to identify food, danger, mates and sensual pleasures such as nature, perfumes.
Smell is detected by sensory receptors called chemoreceptors. When an odorant excites the chemoreceptors, they pass on the electrical impulses to the brain. The brain then interprets these electrical impulse patterns as specific odors. Olfaction happens in a part of the brain that also affects mood, emotions and memory. In fact, it affects cognition and psychoemotional circuitry more than any other sense. Therefore, 'smell' has the ability to affect our moods and emotions. The use of scent to affect mood or behavior is called aromatherapy.
What's perhaps unique about this sense is that it lodges itself in the long term memory system of the brain. The next time you walk into a crowded room and you immediately experience a pang of emotions because you thought you smelled something that reminds you of something or someone, you know the reason why.