Dividing the earth by physical properties?

I am learning earth science and part of my notes say:

The earth, by physical properties, is divided into 3 key regions:
-lithosphere
-asthenosphere
-core

Is the above correct?

I don't understand why the earth is divided into lithosphere, asthenosphere, and core.
And where does the mesosphere go? (lithosphere + asthenosphere + core isn't the "whole" earth)