You know it well: the smell of rain. Some people claim they can smell precipitation before it comes. Whether they can or not is arguable, but practically everyone knows what it smells like after it rains. Some may find this a pleasant smell, others may not.
Interestingly enough, this "rain smell" is caused by a bacteria in the ground. Actinobacteria include some of the most common soil life, freshwater life and marine life. It plays an important role in the decomposition of organic material.
Thus, it also plays an important role in the turnover of organ matter and the carbon cycle. In the soil, this replenishes the nutrients. Some types of Actinobacteria, the Actinomycetes, are responsible for the odor that comes from the soil after it rains, mostly in warmer climates.
This is perhaps what people refer to when they say they can smell rain. The specific chemical that cause this odor is called Geosmin.