Philosophy is difficult to understand, not just for some of the ideas involved, but because the thing itself is so broad. In general, philosophy is the study of the way people think and act, and the study of human beings and what they believe and do. A person who writes and thinks about these topics is called a philosopher.
Aristotle was a Greek philosopher who lived from 384 to 322 B.C. His interests were theatre, music, art, logic, mathematics, politics and ethics. He also invented a version of the "Scientific Method," which you'll study in science class at some point. He was the student of Plato, who was a student of Socrates, and he taught someone special of his own: Alexander the Great.
Rene Descartes lived from 1596 to 1650 in France. He believed in something greater than the human body and was famous for his quote: "I think, therefore I am." He did a lot to influence religious thought, as well as mathematical thought, where his discoveries were important to calculus. Descartes is still taught in every philosophy school to this day.
Immanuel Kant lived from 1724 to 1804 in Prussia. His most famous work is "A Critique of Pure Reason," which questions the limits of human knowledge. He tackles big questions like, "Who created the world?" and "How can we know these things for sure?" His work is very influential in philosophy studies to this day.
Fredrich Nietzsche lived from 1844 to 1900 in Germany. He wrote and talked a lot about the idea of truth, the meaning of tragedy and religions like Christianity, which he ended up rejecting. He was appointed a university chair of Classical Philosophy at the age of 24!
Plato was a Greek philosopher who lived from 427 B.C. to 347 B.C. He was a student of Socrates and helped lay the foundation for Western Philosophy. He did a lot of work in logic and mathematics, as well. His most devoted student was Aristotle.
Jean-Paul Sartre was a French philosopher who lived from 1905 to 1980. In 1964, he was awarded the Nobel Prize in Literature but refused it. He did, a few years later, however, write to the Swedish Academy and ask for the Nobel Prize money in secret. They did not send it to him.
Socrates was an ancient Greek philosopher who was born around 470 B.C. and died in 399 B.C. As a thinker and philosopher, he is a bit of a mystery. You see, Socrates never wrote anything. Everything we know about him comes from his students, who documented his wisdom. He did a lot of work and contributed a lot of idea to ethics, or the system of right or wrong that humans have.