Plato (427-347 BCE) is one of the fundamental philosophers of the European tradition, the earliest of whom a substantial body of work survives. In Greek, his name is given as Platon (Πλάτων). In English, the pronunciation is PLAY-toe, IPA: /'pleɪtəʊ/; the adjective meaning 'to do with, or written by, Plato' is platonic, which has several different meanings.
Plato came from an aristocratic Athenian family and, apart from three visits to Sicily, spent his entire life in Athens. Unusually for an Athenian of his social class, he took no part in politics - alienated perhaps by the Athenians' condemnation of his teacher and friend Socrates. In c387 Plato founded a school or private university in Athens, the Academy, and the remaining forty years of his life were devoted to teaching and research within the Academy. (Many of the next generation of Greek philosophers, including Aristotle, studied in the Academy.)
Unlike Socrates, who wrote nothing, Plato committed much of his philosophical thought to writing - in the form of dialogues, i.e., imaginary conversations in which, rather than setting out and defending his own position, he presents a discussion to which a number of individuals contribute. Prominent among the participants in most of the dialogues is a figure called Socrates but, with the exception of the early dialogues, it is clear that the views expressed by this figure are not those of the real or 'historical' Socrates, but views which Plato himself is inclined to endorse.
Plato effectively defined the nature of philosophy in the western tradition. The questions he asked are the questions with which philosophy is still concerned today, and many of his positions and the arguments he used to defend them are taken seriously by contemporary philosophers.
For a little more about Plato's philosophy see Platonic dialogues.
Many philosophers in the ancient world acknowledged their indebtedness to Plato, most conspicuously the Neoplatonists, of whom the best-known is Plotinus (205-270 CE). Neoplatonism was the dominant philosophical school in the final centuries of the Roman Empire. It exercised an influence on such Christian thinkers as Augustine (354-430 CE) and, later, influenced philosophers in the Middle Ages and the Renaissance.
For some help with the pronunciation of Greek names see Pronunciation of Greek Proper Names.