Biology:

Biology is the branch of science dealing with the study of life. It is concerned with the characteristics, classification, and behaviors of organisms, how species come into existence, and the interactions they have with each other and with the environment. Biology encompasses a broad spectrum of academic fields that are often viewed as independent disciplines. However, together they address phenomena related to living organisms (biological phenomena) over a wide range of scales, from biophysics to ecology. All concepts in biology are subject to the same laws that other branches of science obey, such as the laws of thermodynamics and conservation of mass.
Escherichia coli Tree fern
Goliath beetle Gazelle
Biology studies the variety of life (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle

At the organism level, biology has explained phenomena such as birth, growth, aging, death and decay of living organisms, similarities between offspring and their parents (heredity) and flowering of plants which have puzzled humanity throughout history. Other phenomena, such as lactation, metamorphosis, egg-hatching, healing, and tropism have been addressed. On a wider scale of time and space, biologists have studied domestication of animals and plants, the wide variety of living organisms (biodiversity), changes in living organisms over time (evolution), extinction, speciation, social behaviour among animals, etc.

While botany encompasses the study of plants, zoology is the branch of science that is concerned about the study of animals and anthropology is the branch of biology which studies human beings. However, at the molecular scale, life is studied in the disciplines of molecular biology, biochemistry, and molecular genetics. More fundamental than these fields is biophysics which deals with energy within biological systems. Even at the next level, that of the cell, it is studied in cell biology. At the multicellular scale, it is examined in physiology, anatomy, and histology. Developmental biology studies life at the level of an individual organism’s development or ontogeny. Moving up the scale towards more than one organism, genetics considers how heredity works between parent and offspring. Ethology considers the behaviour of groups of organisms. Population genetics looks at the level of an entire population, and systematics considers the multi-species scale of lineages. Interdependent populations and their habitats are examined in ecology and evolutionary biology. A speculative new field is astrobiology (or xenobiology), which examines the possibility of life beyond the Earth.

It is important to note that the term “life sciences” is beginning to take on a slightly different meaning from “biology.” The life sciences include botany, ecology, evolutionary science, biology, and more. Biology, in turn, tends to include microbiology, biochemistry, biophysics, molecular biology, neurobiology, immunology, molecular genetics, cell biology, and a few other subjects. Biology tends NOT to include botany, ecology, evolutionary science, and a few other disciplines. This division between biology and its allied subjects like ecology should not be seen in parallel to the division between physics and chemistry, but more like the division between physics and astronomy/planetary sciences, where the latter subjects (astronomy/planetary sciences) use concepts from its parent science (physics). The division between ecology/botany/evolutionary science/etc. from biology can be seen most clearly at MIT, Caltech, UCSF, and Rockefeller University, where these institutions don’t even provide any program of education (i.e. major) leading to a degree one of the allied subjects of biology (ecology/botany/evolutionary science/etc.) Although this may change in the future, it can be said that the CORE natural sciences are physics (not including astronomy/planetary sciences/etc.), chemistry, and biology (not including ecology/etc). Even many larger institutions are dividing their faculty members according to this definition of biology (i.e. UC Berkeley, Harvard). Some other institutions do not do this (i.e. USC, Stanford).