neurons are cells which carry messages from the brain to the muscles or the respective area of response. Or in much simpler words neurons carry the stimulus towards the brain or spinalcord and the response from the former. They are also the longest cells in the body. they carry messages from one part of the body to the other by electric impulses induced by micro chemical reactions. Infact they are the most vital cells in the nervous system of an organism.
During the development of the vertebrate nervous system, many neurons become redundant (because they have died, failed to connect to target cells, etc.) and are eliminated. At the same time, developing neurons send out axon outgrowths that contact their target cells. Such cells control their degree of innervation (the number of axon connections) by the secretion of various specific neurotrophic factors that are essential for neuron survival.