Listed below are some properties of living matter that separate it from non-living matter. These properties help scientists distinguish the living world from the non-living world. While it may seem obvious what is alive and what is not, a visit to the microscopic realm quickly blurs the distinction. One of the big reasons for interest in this topic concerns the search for life on other planets. If you want to discover whether life exists elsewhere in the Universe, you need some blueprint for comparison. These characteristics provide that blueprint. Now there is and has been and probably always will be debate over the origins of life; that doesn't concern us here. We're also not concerned with the debates (which rage daily by scientists and non-scientists) over whether these characteristics are valid or not. They give us a good glimpse at what makes living matter special. They help us think about the movement of energy and matter not only in individual marine organisms but also in oceanic ecosystems. They also give us a starting point for considering whether our own planet is a living organism, as proposed by James Lovelock in the Gaia hypothesis.
While this characteristic alone is not sufficient to define life (many non-living forms of matter, such as crystals, exhibit complex structure), it is a prerequisite. Think about the organization of living things. An organism is made up of organs, which are made up of tissues, which are made up of cells, which are made up of organelles (at least in eukaryotic life forms; organelles are little "organs", like the chloroplast, the nucleus, mitochondria, Golgi bodies, endoplasmic reticulum, snips and snails and puppy dog tails...ooops, wrong lecture), which are composed of molecules made of atoms made of particles, which are made up of who knows what!
Homeostasis is the maintenance of relatively constant internal conditions. Human bodies maintain a body temperature that is a rather constant 98.6 degrees Fahrenheit (except for mine, which runs a little cool). Our bodies accomplish this through a series of biofeedback mechanisms, such as shivering when it's cold or sweating when it's warm. Animals such as lizards lie in the sun or the shade to maintain a relatively constant temperature. Not all life regulates temperature, but other properties are regulated, such as their chemical composition, the amount of water in their bodies, their salt content, etc. Crystals and other non-living things don't readily maintain internal conditions.
Most living things with which we are familiar start out small and grow big. The seed in the Styrofoam cup (talked about in All I Need to Know I Learned in Kindergarten by Robert Fulghum), your first puppy, your kid brother or sister; they all grew and developed and changed. The seed got leaves, the puppy's yap changed to a bark and your little brothers and sisters became men and women (or soon will!). Even single-celled organisms grow; phytoplankton, the single-celled plants of the sea, have life cycles. They may expand or contract in size, they may turn into a "seed" called an auxospore or they may get fat and sink into the dark to wait for better times. While you could again use a crystal as an example of a non-living thing that grows and develops, the other requirements for life given here pretty much restrict crystals to the non-living world, at least, in the scientific sense.
Life duplicates. Over and over and over. Rabbits, roaches, mice, humans....these are the more prolific members of the living kingdom and we need no introduction to their modes of reproduction. While some organic molecules (like ribonucleic acids or RNA) in a biochemist's lab can be made to duplicate, they don't exhibit the other characteristics of living matter and, therefore, are not considered alive (yet).
Plants absorb water and minerals and create flowers and leaves. We eat Taco Bell or Denny's fries with Ranch dressing and create strong bodies and minds. (How we accomplish this on that food is one of the still-yet-unsolved mysteries of the Universe!) Life is constantly in motion, orchestrating the flow of energy and matter in a beautiful celebration of what is possible.
You get hit on the head with a brick, you say "Ouch." You respond. Even simple bacteria can twirl their way towards some yummy or twirl their way away from some bad. The list of stimuli and responses goes on and on. You get the idea.
Living matter improves itself. External environmental forces or biological competition for resources favors certain individuals of a given species. These favored individuals typically have slightly different genetic codes than the non-favored types. (All populations of organisms have variations in their genetic codes, including humans.) If those selective forces continue, then the genetic codes of the favored individuals survive and reproduce. Eventually, after thousands, perhaps millions of years, the genetic codes of the survivors are different enough to be considered a new species. Those genetic codes that aren't favored may go extinct (and become fossils.) The fossil record provides unequivocal evidence that living things evolve (or change) over time. The evidence for natural selection is overwhelming and you really should have no trouble accepting it. It doesn't mean that you have to accept that humans came from apes or that God doesn't exist. That isn't what scientists (especially Darwin!) is saying at all. What they are saying is that life changes in response to environmental or biological factors. Genetic characteristics that are favored survive and those that are disfavored pass away. Evolution of galaxies, stars and living organisms is the inevitable consequence of the second Law of Thermodynamics. It really means that the one constant in this world is "change."