Here, I outline the various sources of energy that different life-forms use. The most fundamental division of life from an energy perspective is that between self-feeders that can utilize non-biological forms of energy such as light (autotrophs) and those that feed on other organisms (heterotrophs). Further subdivision of the autotroph category shows that not all of these organisms conduct what can be called ordinary photosynthesis – the type that yields oxygen. Some autotrophs conduct non-oxygenic photosynthesis, while others do not use light at all, but rather utilize chemical energy of various kinds in processes that are collectively called chemosynthesis. Next, I consider the flow of energy in ecosystems and note the limitations in the efficiency of energy transfer between trophic levels, but also the limitations of the trophic-level concept itself. Finally, I note that the biosphere is a realm of decreasing entropy: processes that contribute to this decrease include evolution, embryological development, and ecological succession. The decreasing entropy of the biosphere is perfectly compatible with the second law of thermodynamics as this law only applies to closed systems.