Biology encompasses several fields of study about the living organisms and their environment. Disciplines range over several levels of organization, such as atomic (biochemistry, structural biology, and genetics), microscopic (cellular biology), organismal (botany, zoology, biophysics), and large-scale levels (ecology, evolution, and agriculture). Human-related fields tend to be the most applied, such as genetics, physiology, and medicine. Despite having multiple subdisciplines, all fields of biology are deeply related to each other.
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. Cell Theory. Cell theory, as put forth by Schwann and Schleiden in 1838, stipulates that all life is made from cells, that the cell is the fundamental building block of life, and that all cells come from preexisting cells.
2. Genetic Theory. Determined by Griffith in 1928, DNA is the genetic material of all cells. DNA encodes genes, which are transferred from one generation to the next and determine the characterisitcs of an individual.
3. Homeostasis. All living organisms attempt to reach of dynamic equilibrium with their environment - from chemical equilibria, to cellular homeostasis, to organismal regulation, to population dynamics.
4. Change. While evolution is debated to be a core tenant of biology, short-term changes do occur in populations that are important for ecological and genetic interactions that occur. Change is also important in development, population dynamics, mutations, and homeostasis, where nothing remains static and everything is in constatn flux.
More than ten thousand years ago, Mesopotamia showed evidence of the domestication of goats, sheep, and grains. While most likely unintentional, it can be placed as the first point at which human study of the nature of life itself began.
The discoveries of the nineteenth century spurred the development of modern biology. Darwin's observations began the controversial field of evolution, Mendel's breeding experiments was the start genetics; the microscope opened microbiology for exploration; and the synthesis of urea initiated biochemistry.
In the early twentieth century, statistics, in tandem with the new fields of evolution and genetics, spurred the development of population genetics. 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 biology.
Modern biological research is heavily focused on identifying genes and characterizing their expression and function. Structural biology has been vital for understanding diseases and health defects. Ecology has been advancing to better estimate the biodiversity on the planet as well as how to prevent extinction events
- Martin, E. and Hine, R. (2008) 'A Dictionary of Biology' Oxford University Press, Oxford, U.K.