In school we all were taught the difference between living and non living things. Whatever could move, talk and display emotions was a living thing and something which was stable, inert, or didn't talk was a nonliving thing, this was the basic difference I was taught by my parents and teachers when I was a kid. It was so easy to differentiate the things as living and non living with these criteria of differentiation. But even then I had some confusion in mind; a flowing river, it has to be living because it moves and also makes noise. Plants, they never say anything nor move, aren't they non living things? So, do we really have adequate knowledge to be able to distinguish these two categories? Because, I have again got some unanswered questions in my mind. The invisible destructive virus, what is it, a living thing or a non living thing? A person in coma due to a severe head injury is referred to as brain 'dead', so how can the person be called 'living'? The comparison between living and non living things is not easy as it seems on a first note. Thus, in my quest to understand the minute differences between these two categories, I have summarized the differences that classify a thing as either living or nonliving.

Point of Comparison	Explanation
Respiration	Respiration, that is the process of breathing is the key criteria that separates a living being from a non living being. The process of respiration is same in almost all living beings, which generally is taking in oxygen and exhaling carbon dioxide. Humans and most other mammals obtain oxygen through the lungs. Some aquatic animals like fish take in oxygen through their gills. There are some animals that breathe through their skin. But the process of respiration is slightly different in plants; instead of taking in oxygen they take in carbon dioxide, and exhale oxygen. The concluding point is that respiration is a key process of survival, be it any living being.
Life Processes	Almost all living beings exhibit life processes like metabolism, homeostasis and genetic mutations. There are many complex processes that take place in a living organism's body. On the other hand, non living things do not undergo these life processes. All the living organisms have the ability to reproduce and give birth to organisms of their kind.
Response to Stimuli	Living things show response to stimuli. Humans, plants, fish and animals, all respond to the changes in the surroundings. Non living beings on the other hand would never show response to the stimuli.
Death	Living beings follow a life cycle that progresses as birth, growth and death. Death is imperative for any living being; non living things would never have to face death. They can be destroyed due to natural calamities or some actions, but they never die.
Composition	Cells are the basic constituents of all the living organisms. Plants and animals all are made of countless cells. Cells are essential to perform the basic processes in life. There are number of processes that undergo in living organisms. Non living things on the other hand lack the chemical processes and are composed of matter that doesn't undergo any chemical change on its own.

As we had discussed earlier, there is confusion to distinguish certain entities as living or non living. I need to mention that there have been varied opinions to classify these entities. Let's find what science opines about these two categories... Virus: Living or Nonliving? Many people consider that viruses are living. The reason people hold this opinion is that there are many viruses that cause diseases and thus cause destruction. But this understanding is wrong. Viruses actually are non living things. They cannot do anything on their own and require the help of the cells in the body of a living organism to multiply and cause the destruction. The destruction causing virus is rendered useless if it has no host. A Brain Dead Person Science holds the opinion that a brain dead person (if the entire brain is dead) has actually died; because, the inability of the brain to function also hampers the breathing and other bodily functions in the organism, that characterize life. A brain dead person also doesn't show other common characteristics of life like response to stimuli. Religious groups and certain people find this opinion of science quite difficult to accept, and there is still a debate to characterize a brain dead person as dead or living. All these concepts, though apparently simple, are difficult to understand. Hopefully, these concepts have helped you understand the complex concepts of living and non living things.