Defining life is tricky because of the wide variety of organisms that exist on Earth. However, there are some basic, major functional characteristics of all organisms that distinguish them from inorganic materials. Among these important distinctions are reproduction, evolution, a need for food and specialization of tasks in the organelles of cells.
One of the most important functional characteristics of all organisms is that they must posses an ability to reproduce and create more of the same organism. Animals reproduce sexually; simpler, single-cell organisms reproduce by division through the process of mitosis. Plants reproduce either sexually or asexually. The information needed for an organism to create new organisms is encoded in DNA and RNA; in multicellular organisms, the cells that create new organisms are produced through a process known as meiosis, which involves splitting the genetic code in half so that it can combine with the information from another organism.
All living organisms must subsist on some form of food source in order to give them energy to power their functions. Organisms that make their own food internally are known as autotrophs. All plants and several types of bacteria and protists are autotrophs because they make their food using sunlight, water and carbon dioxide (a process known as photosynthesis). Organisms that cannot make their own food are known as heterotrophs. Heterotrophs gain their energy by consuming other organisms.
Another important characteristic of an organism is the ability to evolve and adapt based on its surrounding. This process of evolution is directly affected by natural selection, which ensures that organisms that have developed helpful features survive long enough to pass those features down to their offspring. For example, flowers have adapted bright colors to attract bees so that the bees will spread their pollen, and all terrestrial-based animals have adapted lungs from gills so that they can breathe on the surface.
Organisms also have cells that help divide the tasks which the organism needs to do in order to survive. Most bacteria are classified as prokaryotes because they are relatively simple cells that lack a cell nucleus; all other organisms are classified as eukaryotes. Important organelles in the cell include chloroplast in photosynthesizing organisms (because chlorophyll in the the chloroplast is what captures sunlight) and mitochondria in plants and animals, which contains the DNA and RNA.