Earth has four spheres which are the building blocks of our world. The Atmosphere is the air and the gasses that make it up. It has five different layers that all do different things, Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere. The Geosphere (or Lithosphere) is the rocks and other pieces of solid Earth. The Biosphere is all of the living things that we see, or may not see. It can be anything from the tallest tree to the smallest bacteria, so we are part of the Biosphere. Lastly there is the Hydrosphere, and this includes all of the water on Earth. This includes rivers, lakes, and oceans, but also frozen water in glaciers and all of the moisture in the air and ground.
A picture from Crash Course showing the four spheres
(original image)
I think that each individual character in this show can be placed into categories based on the four spheres. Monica is in the Geosphere category because she is like the rock that holds the group together. Ross is in the Biosphere category because he is a paleontologist, so he works with life and things that used to be living. Rachel and Chandler are apart of the Hydrosphere category because they both are able to go with the flow. Phoebe and Joey are apart of the Atmosphere because their heads are always in the clouds, and they are never really focused on the conversation.