Air is the mixture of colorless gases that make up the earth's atmosphere. It's about 78% nitrogen (N2) and about 21% oxygen (O2), with the remaining 1% made up of carbon dioxide, argon and other gases. Air also contains varying amounts of water vapor, pollutant gases and suspended particles. Air exhibits the typical characteristics of gases: it's compressible and doesn't have a definite shape.
The weight of the air above us is what creates atmospheric pressure. At higher altitudes the air pressure is lower because there's less overhead compared to lower elevations. The air is therefore less dense and can cause those not used to high altitudes to feel out of breath when engaging in physical activities.
All things living on land need air to survive. Animals get oxygen and plants get carbon dioxide from air.