Biology encompasses several fields of study, including genetics, biochemistry, cell biology, structural biology, mammalian physiology, biophysics, medicine, botany, and zoology; in addition, studies such as ecology and evolution also fall under the purview of biology.
An Oxford University book states:
It is 'the study of living organisms, which includes their structure (gross and microscopical), functioning, origin and evolution, classification, interrelationships, and distribution.' 
The study of biology works from several basic foundations:
1. The cellular theory of life. All life is composed of at least once cell, which undergoes the classical processes of cellular life, such as reproduction; the cell is the basic unit of all macrobiotic life. The chemical composition of all cells in nature is similar, and all existing cells have emerged from prior cells through the processes of cellular division, generally through mitosis but with meiosis playing a significant role.
2. Genetic theory. All cells encode their genetic information in the form of DNA, the fundamental component of genes. These genes transfer the physical, and often psychological, traits of one generation of organism to the next.
3. Homeostasis. All living organisms will attempt to reach some form of dynamic equilibrium with their environment - both through the metabolism of individual creatures, and the population dynamics of whole populations.
More than ten thousand years ago, Mesopotamia showed evidence of the domestication of goats, sheep, and grains; while this was most likely unintentional, at least at first, it can be placed as the first point at which human study of the nature of life itself began. From these humble beginnings, all of modern biology eventually flourished.
The discoveries of the nineteenth century spurred the development of modern biology. Mendel's breeding experiments initiated genetics; the microscope initiated microbiology; and the synthesis of urea initiated biochemistry. In the early twentienth century the field of population genetics spurred the development of statistics. Advances in analytical chemistry permitted scientists to determine the structure of important biological molecules, such as DNA. These discoveries lead to the rapid development of molecular genetics. Coming full circle, these developments, combined with population genetic theory, is permitting scientists to examine the process of crop and animal domestication.
Modern biological research is heavily focused on identifying genes and characterizing their expression and function. Progress in this field has enabled scientists to analyze organisms as complex molecular systems. The study of complex systems has also been applied to ecology. There is also rapid characterization of the biodiversity of the Earth. This is especially pronounced in microbiology, where surveys of diversity have only become possible with the development of modern molecular techniques.
- Martin, E. and Hine, R. (2008) 'A Dictionary of Biology' Oxford University Press, Oxford, U.K.