Q. Define Central nervous system?

The nervous system begins as a simple tube during embryonic development (then anterior part expands and also ventricles are formed). 

Forebrain -> L. and R. cerebral hemispheres (subdivided into frontal, parietal, occipital, temporal lobes).

These centers include the cerebral cortex and are responsible for motor skills, perception, thinking, memory, speech, ...and other cognitive functions.

Diencephalon -> Thalamus and Hypothalamus. 

The thalamus acts as a major relay station for sensory input.

Hypothalamus regulates autonomic functions (e.g. body temp., water balance, metabolism, appetite, sex drive, ...)

Brain Stem -> Midbrain, Pons, Medulla Oblongata and Cerebellum.

Midbrain:  reflex centers for hearing and vision (occulomotor nerve)

Pons:  main center for control of breathing

Medulla:  regulate vital visceral activity (e.g. HR, BP, breathing, swallowing, sneezing, vomiting,..)

Cerebellum -> precise control of motor skills and spatial positioning (fine tuning of skeletal muscles/ Proprioreception).