Best known as author of the 1859 scientific text "On the Origin of Species (by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life)", Charles Darwin has become a controversial fixture in discussion and debate regarding biological theory. His name has been affixed to many ideas, the most prominent of which is the relatively ill-defined "Darwinism." The term is used frequently, yet few understand its exact meaning, scientifically speaking. This all begs the question: What is Darwinism?
First, it is necessary to make the distinction between the Darwinism of physical science, which is the subject of the given question, and Social Darwinism. Social Darwinism is a forerunner of eugenics that applies certain aspects of the theory of natural selection to human society to derive the idea that, as with populations, individual human beings succeed or fail based on their own merits. This philosophy is not true Darwinism.
Darwinism is now one of several versions of the evolutionary concept. However, it is not to be confused with the current accepted and most developed evolutionary theory. Darwinism is taken to rely largely on natural selection and encompasses theories from other scientists in addition to the work of Charles Darwin himself.
The definition of Darwinism has changed over time. In Darwin's day, it was used interchangeably with "Social Darwinism," and it was really more of a symbol than a specific concept. Darwin's legacy has impacted many aspects of the (especially Western) world, and many revolutionary ideas fell under the banner of Darwinism regardless of their connections, or lack thereof, to his actual work.
In the United States, Darwinism is often seen as a popular pejorative term among creationist groups in a fallacious attempt to identify modern evolutionary theory with the (also incorrect) idea that Darwin claimed that humans were essentially direct descendants of monkeys and/or apes. As evolution is still controversial, even a good century and a half after The Origin of Species was first published, the term "Darwinism" allows opponents to greatly oversimplify and, at times, vilify the concept.
At its core, though, Darwinism is simply an aspect of evolutionary theory that deals most especially with natural selection. It is not the same as the modern accepted theory of evolution due to the fact that Darwin had little knowledge of heredity and genetics; evolutionary theory was essentially updated after the work of Gregor Mendel, who is famous for his work with crossing pea plants. The importance of Mendel's research to evolutionary theory was not realized for decades though, and Darwinism logically does not reflect the genetic aspect of evolution.
"Darwinism" means many things to many people, but it stands first and foremost as a scientific concept emblematic of innovation and revolutionary thought.