Living matter is organized into complex structures based on organic molecules

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!

Living matter maintains some type of homeostasis
Homeostasis is the maintenance of relatively constant internal conditions. Human bodies maintain a body temperature that is a rather constant 98.6 degrees Fahrenheit. 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.

Living matter grows and develops
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. 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.

Living matter reproduces and passes on genetic material as a blueprint for growth and subsequent reproduction.
Living matter acquires matter and energy from the external environment and converts it into different forms
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.
Living matter evolves
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."

1. A population is a summation of all the organisms of the same group or species, which live in a particulargeographical area, and have the capability of interbreeding.

In ecology, the population of a certain species in a certain area is estimated using the Lincoln Index. The area that is used to define a sexual population is defined as the area where inter-breeding is potentially possible between any pair within the area. The probability of interbreeding is greater than the probability of cross-breeding with individuals from other areas. Under normal conditions, breeding is substantially more common within the area than across the border.

In sociology, population refers to a collection of humans. Demography is a social science which entails the statistical study of human populations. This article refers mainly to human population.