What is CO2?

At room temperatures, carbon dioxide is a naturally occurring, colorless, odorless gas.
When inhaled at concentrations much higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat.
It is used by plants during photosynthesis to make sugars, which may either be consumed again in respiration or used as the raw material to produce cellulose for plant growth.
It is a major component of the carbon cycle.