What is Biology? Biology is the science of life, or rather, the intersection of living organisms and their environment.
What is life? Life is anything that possesses all seven properties outlined below.
1. Homeostasis. Homeostasis is a process in which a system maintains balance. Living organisms I have a tendency to maintain a balance. If they didn't, either they would be spending too much energy and run out of energy. or they would be only absorbing energy and never spend energy so not able to do anything.
2. Organization. Living organisms are said to be organized (or rather, more organized than the environment in which they live in) in some fashion. The complexity of that organization varies. The basic unit of life (therefore the basic unit of organization) is typically considered to be a cell. There's a variety of types of cells that could be less complex or more complex. For example, bacterial cells or rather, prokaryotic cells, are considered to be less complex because they lack a nucleus and organelles. Eukaryotic cells (plants, animals, fungi, and others) are considered to be more complex because they have a nucleus and other organelles, which are compartmentalized structures within a cell that perform specialized functions. Both types of cells are organized. In multi-cellular organisms, cells are organized into tissues, and tissues are organized into other organs so on and so forth, up through the organism.
3. Metabolism. Metabolism is the using of energy, so living organisms are thought to transform energy in some way. Not only does it collect energy and spend energy, it transforms that energy. So when an organism either eats food, or collects sunlight, or some type of energy-gathering function, it doesn't simply spit back out the energy in the same form as it was originally collected (e.g. collecting sunlight, emitting sunlight). It changes that energy, and at the same type uses some of that energy to maintain homeostasis and any of the other properties of life that require energy input. That's a special property of metabolism, the transformation of energy.
4. Growth. As living cells and organisms continue their life cycles, they will use the properties of organization, metabolism, and homeostasis to obtain energy, transform that energy into structural components and then, at some point, at the cellular level the cell will divide creating a new cell, or at the organismal level, the organism will somehow make a copy of itself (either asexual or sexual reproduction). At the cellular level, it will accumulate energy and resources until it no longer can continue to accumulate those resources, and that cell will divide. At the organismal level, for example, plants and animals will maintain some excess resources for energy storage during times when there might be food shortages, but typically excess energy will go into the development of reproductive structures (e.g., flowers and fruit in plants) or invested into the next generation (e.g., eggs in reptiles and birds).
5. Adaptation. The fifth property is adaptation. When living organisms grow, they divide and/or reproduce. During that growth process, there's a transfer of information and that information is stored in DNA as the genetic component of life. Darwin considered there to be some heritable component transferred from one generation to the next, but he didn't necessarily know what genetics or DNA were, but he knew that organisms inherited some piece of information. Now, in between generations, that information changes slightly through random chance. In bacterial cells, when the cell divides, it begins to make a copy of its genome. When the molecular machinery in the cell begins to copy that DNA, it might make a mistake. And so instead of an ATG in the genetic code, it might have been ATC now, and this could either cause a negative change in the next generation or a positive change. And if it's a positive change would be considered an adaptation, so adaptations are considered positive, heritable changes that helping organism to survive either in its current environment or in a new environment. And this is the fundamental process of evolution.
6. Response to stimuli. Living organisms will have some type of response to stimulus that could be even the most basic response, for example, in bacteria that could be a response to chemical signals. Bacteria might move along the surface of a pond or of an animal's skin in search of a chemical that it considered that is food. So even the most basic organisms have some response to stimuli, even if it's just a chemical response. It might also be something more advanced, like the behavioral response to a food-source found in animals. In animals, we might think of a worm being incredibly more complex then a bacteria because worms might be made up of hundreds or thousands of cells, but they still might only have a simple response to chemicals. The range of response-to-stimulus complexity goes all the way up to humans, which are respond much more complex ways.
7. Self-reproduction. Self reproduction is one of the most important parts of defining whether something is alive or not, but it's also one of the most controversial because now a virus could be excluded from the definition of a living organism. Cellular life is considered to undergo self-reproduction when it either divides or undergoes sexual reproduction. Viruses cannot self reproduce, it can only reproduce with the help of a cell. So viruses are generally not considered to be alive, although they are considered to undergo evolution processes.