What is grey matter and white matter ?

One of the important  and vital questions of nervous system:
what is grey and white matter ?
Before explaining this we need to know about a neuron
  A neuron has two part
cell body and
processes : dendrites and an axon

if we arrange several neuron in single layer like following picture, we see cell body of neuron make a layer and processes of neuron makes a layer
Both this layer contain neuroglia and blood vessels

So, composition of grey and white matter is : 

•Gray matter:  Neuron cell  bodies

                            Neuroglial cells

                            Blood vessels

•White matter:  Processes  of neuron

                             Neuroglial cells

                             Blood vessels

Location of gray & white matter in the CNS

•Cerebrum and cerebellum ,

•Outer gray matter

•Inner white matter

•Spinal cord,

•outer part white matter

•Inner gray matter

  Why location of grey and white matter differ in different parts of CNS.