The senses are the means by which a living organism perceives a particular type of stimulus from the environment. Traditionally, five human senses are defined: sight, hearing, touch, smell and taste.
The nervous system contains particular organs (the sense organs) which have the function of receiving and responding to these environmental stimuli, and transmitting information to the brain.
For example, the eyes respond to visible light and transmit information to the brain via the optic nerves.
If we think a little more deeply, however, we realise that the environment from which we obtain information includes the internal environment of our own bodies, and we also have more than five senses ...
How about balance? Or pain? Or a sense of direction? Or time?
Different organisms also have different senses: some fish can sense electrical impulses through water, for instance, and some plants can respond to the presence of particular chemicals in air, or in soil.