There are four main layers in the atmosphere, the troposphere, stratosphere, mesosphere and thermosphere. "Sphere" means something that's round, like the Earth itself. Each layer wraps the earth in certain gases. The borders that mark the change between atmospheric layers are called "pauses." The border between the troposphere and the stratosphere is called the tropopause. Next comes the stratopause and the mesopause. There is no boundary between the thermosphere and outer space, called the exosphere.
The main gases that make up our atmosphere are oxygen, nitrogen, argon and carbon dioxide. Oxygen is what we breathe, while carbon dioxide is what plants need to live. While these gases want to rise, gravity pulls them down toward earth. The stronger gravity is, the thicker the atmosphere becomes, as more molecules are pulled toward the Earth. As the weather gets hot, the lower layers of the atmosphere expand, or grow; when the weather gets cold, they contract, or shrink.
The troposphere is the layer of air that you live in and extends 7 to 11 miles above the surface of the planet. It has the most oxygen of any layer, which is why we can breathe down here but not up in space. Even if you climb to the top of a really high mountain you'll have trouble breathing. Also, the higher up you go, the colder it gets. Hot air rises then becomes cold, which makes moisture form clouds then turn into rain, and air to fall again. This causes wind and other weather phenomenon.
Large airplanes usually fly in the lower stratosphere. It starts after the tropopause and continues on up to about 20 miles over the Earth. The stratosphere is also where most ozone is; ozone is different kind of oxygen molecule that absorbs radiation from the sun, keeping us all from getting sunburned every day. After that comes the mesosphere, which stretches to about 50 miles, then the thermosphere, which mixes gradually with the exosphere.