Socrates was a Greek philosopher and is seen by many as the founding figure of western philosophy. There were philosophers before Socrates (referred to as the pre-Socratics) but no philosopher, before, touched upon the diversity of topics that Socrates did. Socrates did not write anything and we know of him only through the dialogues of his student, Plato, and through the other 'Sokratikoi Logoi' (discourses of Socrates) written by other students such as Xenophon.
Socrates was a citizen of the city-state of Athens and spent most of his life there until he was sentenced to death in 399 BCE. He was a fairly old man by then. His trial and death are discussed in three Platonic dialogues: Apology, Crito, and Phaedo. In the Apology he accepts to die rather than to stop asking questions and serving as a 'gadfly' to the Athenian citizens. Plato depicts him as a martyr for philosophy and rational inquiry in general. He held heterodox political and religious views and was charged with not believing in the gods and with corrupting the youth of Athens. He was sentenced to die by ingesting hemlock.