In biology, the term "process of evolution" refers to the way in which organisms change both within species and from one species to another. The process of evolution is also called the process of natural selection.
The way that this process works is that various individual organisms have random changes in their genes that make them in some way different from other organisms of the same species. For example, one animal might have a slightly longer neck than the others. If this mutation allows these individuals to have more success surviving and reproducing, it is passed on to future generations more reliably than other mutations.
Over long periods of time, these mutations that are beneficial can add up until the average member of the species has changed or even until a new species has been created.
Evolution is the constant state of change of any living organism regardless of whether it is a tree, an animal, or a human being. As beings react to their environments certain traits get passed along through genes that enhance the possibility of survival. There are several trains of thought on why this happens, and the most popular this day and age is that of natural selection. Essentially what happens is that the physical attributes, and beahviour patterns in any given living organism that are most beneficial to survival continue on through the genes because those living organisms possessing those traits are more likely to survive and pass them on to their offspring. The better a species is at adapating to changes, especially environmental changes, the longer that species is likely to survive over time.
Another process of evolution is that of extinction. By that I don't mean of a species, but of physical attributes and behaviour patterns in a species that become obosolete as a species evolves. For instance, in x-rays of whale flippers one can see the remains of what look quite like human hands. As the whale ancestor spent more and more time in the water, these "hands" became less necessary and so overtime they were absorbed into the body.
In common language evolution means any slow change process taking place over a long period of time. In science this term is used most commonly to refer to the process of origin and development of life on earth. It is also used to refer to the process of evolution as per the theory of evolution proposed by Charles Darwin in 1858, which holds that all species of plants and animals on the earth evolved from a single form of of life. Over time this basic form of species evolved into other kinds of species due to chance variations in the original species. These other form of species still developed in further kind of species. Many times the other species that developed and survived were more evolved than the species that gave rise to them. This process of development of one kind of species into any other continued many thousands or millions of times over, giving rise to a millions of different forms of life. Many of the species thus produced became extinct because they were not that well adapted for survival in the environment in which they developed. Others developed well to their environment and survived. This process of evolution has produced more than 2 million species that inhibit the earth today.