A Philosopher is someone who engages in philosophy. Philosophy comes from the Greek word 'philosophia', which means, 'love of wisdom'.
In the Western European tradition, this is generally thought of as beginning with the Athenian philosophers of ancient Greece (Hellas); Socrates, his pupil Plato and Plato's pupil Aristotle. This group of philosophers constitute the core of what are known as the 'ancients', and their systems for understanding the universe from the rationalist (as opposed to revealed scriptural) perspective reigned generally until the Middle Ages.
The Renaissance that followed the Middle Ages partly consisted of a rediscovery of this ancient teaching, and resulted in a new wave of philosophers known as the 'moderns'. This group followed in the wake of the Copernican revolution in physics that the earth was not the center of the Universe, as had been assumed by the ancients. French philosopher Descartes pronounced "I think therefore I am", challenging God's statement that "I am that I am".
It was at the end of the 17th Century, however, that modern philosophy began, during the 18th Century 'Enlightenment'. Englishman John Locke founded the school of 'empiricism'; that information not gleaned from the experiential senses is of no value. Scottish philosopher David Hume became the chief philosopher of skeptical atheism, stating that it is never rational to believe in miracles. Finally perhaps the greatest work of modern philosophy was produced by German scholar Immanuel Kant in 1781, the 'Critique of Pure Reason', in which he argued that although reason can be used to understand the world around us to a qualified extent, it can never be absolutely relied upon, as we can never have access to absolutely certain information from our senses. This book arguably gave birth to the romantic era.
In the Victorian age notable philosophical works were produced by German Friedrich Nietzsche, an arch-modernist and atheist who claimed 'God is Dead'. In the 20th Century philosophy branched out into multiple specializations. The philosophy of science by Karl Popper, the philosophy of language by Ludwig Wittgenstein, the avant garde modernism of Jean Paul Sartre's 'existential' school, and in the 1970s the attempt to 'deconstruct' modernism altogether with the postmodern philosophy of Michel Foucault and Jacques Derrida.
Probably the most important and influential philosophical work of modern times however, commonly thought to be a natural science work, is Charles Darwin's 'Origin of Species', that attempted to raise the regard for human knowledge above the teachings of God in scripture. This philosophy, based in part on eastern mysticism, provided the intellectual foundation for Nietzsche, and the fascism and communism of the 20th Century.