Neo-Platonism is an ancient school of philosophy beginning in the 3rd century A.D. It was based on the teachings of Plato and Platonists; but it interpreted Plato in many new ways, such that Neo-Platonism was quite different from what Plato taught, though many Neo-Platonists would not admit the distinction.
Neo-Platonism began with the philosopher Plotinus, though Plotinus claimed to have received his teachings from Ammonius Saccas, an illiterate dock-worker in Alexandria. His most important work was the Six Enneads, in which he explains his philosophy.
Plotinus taught the existence of an indescribable One, which emanated the rest of the universe as a sequence of lesser beings. Later Neo-Platonic philosophers, especially Iamblichus, added hundreds of intermediate gods and beings as emanations between the One and humanity; but Plotinus' system was much simpler in comparison.
Neo-Platonism was frequently used as a philosophical foundation for paganism, and as a means of defending paganism against Christianity; but many Christians were also influenced by Neo-Platonism. In Christian versions of Neo-Platonism, the One is identified as God. Most important of these was Pseudo-Dionysius the Areopagite, whose work was very influential in the Middle Ages. Augustine of Hippo converted to Christianity under the influence of Plotinus, leading most scholars to label Augustine a frank Neo-Platonist; although, they note that Augustine's subordination of philosophy to scripture leads to striking differences from the non-Christian philosophy. Some scholars have shown that Neo-Platonism was also influenced by Christian theology, notably through the belief systems known as Gnosticism.