Socrates was a Greek philosopher born in Athens in 469 BC and sentenced to death (and killed) as the result of a trial occuring in 399 BC.
He himself left no writings, but instead is famous for the influence he had on people with whom he conversed. He had many followers, of whom perhaps the most important was Plato.
We have accounts of Socrates from three of his contemporaries, Plato, Xenophon, and Aristophanes, as well as several later sources (including, most significantly, Diogenes Laertius)
Aristophanes, the oldest of the three sources, was probably acquainted with Socrates when Socrates was relatively young. In 'Clouds', Aristophanes portrays Socrates as a sophist, concerned both with natural philosophy and verbal quibbling. Although the play is a comedy, and probably exaggerates, it may be an accurate portrait of Socrates' youth.
Xenophon portrays Socrates as a traditional moralist, opposed to the excesses of the Greek democracy, and concerned with religion, education, and ethics. No trace of metaphysical or physical speculation is found in the Xenophonic Socrates. Xenophon was travelling outside of Athens during many of the crucial events in his account, and thus there are questions as to the accuracy of his portrait.
Plato's Socrates is the most complex and profoundly philosophical. Plato was a close follower of Socrates, and his dialogues portray Socrates as a profound and complex thinker on ethics and metaphysics. Since the dialogues of Plato were not transcripts of actual conversations, there is a question of whether his Socrates is an accurate portrait.
What we do know for certain is that Socrates had a significant effect on his followers and that the questions he asked are so insightful that philosophers even now continue to debate them.