The troposphere is the layer of the atmosphere closest to the earth. It extends from the ground to approximately 12km above sea level (though this varies with latitude and with the seasons). It contains around 75% of all the air in the atmosphere, and around 99% of water vapour. Most of the weather we experience (wind, clouds, rain) occurs in the troposphere too.
The troposphere is relatively hot at the bottom and gets cooler the higher you go. The stratosphere is relatively cold at the bottom and gets hotter the higher you go. The temperature difference between the troposphere and the stratosphere is caused by UV light passing through the ozone layer and triggering exothermic reactions. The point where the troposphere ends and the stratosphere begins (also known as the tropopause) is the altitude at which temperatures no longer decrease as height increases.
The air in the stratosphere is incredibly thin and very dry - humans would absolutely not be able to survive by breathing this air. In fact, the stratosphere contains the Armstrong Line. This line lies at 19km above sea level and represents the altitude at which atmospheric pressure is so low that water evaporates at body temperature. If a human were to exist above the Armstrong Line outside of a pressurised suit or aircraft, the saliva coating their tongue, throat and lungs would begin to boil. Above this limit, no amount of breathable oxygen delivered by any means will sustain human life for more than a few minutes.
When the stratosphere becomes the mesosphere, the temperature begins to decrease as altitude increases - much like it did in the troposphere. In fact, our atmospheric layers are entirely defined by these abrupt temperature changes.
We spend almost all our lives in the troposphere, but it might surprise you to hear that you may have been to the stratosphere without even realising it. Many commercial airliners choose to fly in the lower stratosphere to avoid the turbulence caused by clouds and convection currents in the troposphere below.