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Air is a mixture of gasses that compose the atmosphere. When the earth was forming, 

air as we know it today did not exist. In the beginning of the earth's formation, gravity drew in materials that were orbiting around the sun. This included gases.Early attracted gases were probably helium and hydrogen. Both are light gases, and it is thought that gravity at that time was not strong enough to hold them around the earth. As a result, there are smaller amounts of these gases in the air today in comparison to the larger amounts found throughout the universe.

We breathe in oxygen from the plants and breathe out carbon dioxide for the plants, 

while plants take in carbon dioxide from us and give off oxygen for us.