That's a bit of a heavy question.  But, for biologists, we are able to break things down into living and non-living things.  For example, rocks are not considered living things but humans are!  In order to be considered a living thing organisms need to be capable of carrying out ALL THE CHARACTERISTICS OF LIFE.  They're listed here.

• Made of one or more cells

• Displays organization

• Grows and develops

• Reproduces

• Responds to stimuli

• Requires energy

• Maintains homeostasis

• Adaptations evolve over time