The sense of smell can also be known as olfaction. This has to do with when someone inhales or exhales. It is regular for people to inhale and exhale 20,000 breaths each day. You smell something when your receptor cells reach about 5 million. Their in a cluster and are carried to the top of your naval cavity and once there they alert the brain through the axon fibers.
The first thing that happens to trigger a smell is the messages from the receptor cells are sent to the brain's olfactory bulb. Then onto the temporal lobe where the primary smell cortex lives. After that to the limbic system where memory and emotion are involved. After this the aroma of whatever you were smelling is enhanced.
The sense of smell is different from the other senses. Odors are all individual. Each scent cannot be separated into separate categories or more elemental odors. There are however many different combinations of the receptors that combine to distinguish scents. Example: hours old coffee or fresh brewed.
Humans sense of smell tends to decline after early adulthood. Odors can also help someone remember a memory or feeling they once had. The area of the brain given for olfaction can also be involved with memory storage, thus meaning why a memory can be triggered by smell.

(Myers, 2007)