Sophist is an early Socratic dialogue. Where it fits in the chronology is a bit unclear. It seems as if it was to be part of a trilogy: Sophist, Statesman, Philosopher, but only the first two still exist. The dialogue begins the day after Theaetetus (wherein the doctrine of justified true belief was expounded) with the interlocutors returning to be questioned by Socrates. Unfortunately, they are out of luck: Socrates himself only takes part in the very beginning of the dialogue. The dialogue itself is performed by Theaetetus, the young, dashing mathematician-philosopher, and an unnamed foreigner. Since the ancient Greek word for foreigner (or stranger) is ξενον, I've taken to calling him Xenon, like the element.
The first part of the dialogue attempts to answer the question "What is a Sophist?" using what was then the novel method of discrimination. This amounts to a very long, drawn out analysis of the skill of sophistry starting from the broadest possible category of the arts (including skills) and repeatedly subdividing categories until the art of the sophist is made manifest. Six such "deductions" are made, from "a paid hunter after the young and wealthy," to "a purger of souls, who removes opinions that obstruct learning". The details are not particularly enlightening, save for an interesting passage on the nature of knowledge.
In this passage (230b-231a), Xenon relates a myth of knowledge as a purification of the soul. This myth plays a huge role in Phaedo, Meno, and The Republic, though whether or not it was developed here or elsewhere is difficult to discern. From this myth we also get the first justification for the use of the dialectic, a justification that is also developed in the above sources: that the method of questioning removes impurities (that is, false beliefs and prejudices) present in the soul so that it can understand things properly.
And, if you're like me, blindly believing everything Plato has been spoon-feeding you so far, everything is working out precisely as planned. But there's far more to this dialogue than meets the eye. With characteristic irony, Plato has described by demonstrating what it is the Sophist does in the very act of appearing to seek the skill of sophistry! It is the Sophists that make seemingly endless distinctions, only to identify the end products. No, my good friend, it is not as Xenon says. The Sophist doesn't fit into more than one slot: it is that the Sophist wants to fit himself and everything else into a multiplicity of slots. God, Plato's such a mindfucker.
It is clearly in Plato's interest to come out and defame the Sophists as liars and tellers of half-truths, but unfortunately, there is metaphysics afoot. The pre-Socratics developed some of the most naive, helpless-looking metaphysics in the western tradition, and thankfully we don't have to give half a second's time to any of them because Plato destroys them all. Truly, my friends, all you need is dialectic.
But perhaps I'm getting ahead of myself. To call a Sophist a liar is to say that there are liars: people who say something that isn't real. But Parmenides said oh-so-tautologically "Being is, and Not-Being isn't" (through the voice of a goddess, so it has to be true). "Well fuck, what gives?", says Theaetetus. So Xenon proceeds to troll some other pre-Socratics, drag their discourse into the dialectic, and tear them to shreds.
Some people, by the way, believe that one of the positions refuted here is one of Plato's early idealist theories, before he came up with Platonic Realism. If that's the case, perhaps there is a shred of humility in the bastard.
After he's run out of naive metaphysics to refute, Xenon makes the huge mistake of trying to set up his own, based on five categories: Being, Motion, Rest, Self, and Other. Using these he cobbles together an argument for what I can tell you in a single sentence: "Not-being is different from non-being." Well, okay, that's totally opaque.
Not-being is impossible to work with, as Parmenides brings up, because we can only speak of things that exist. But Plato argues that the apparent opposition between being and not-being is a false one, and that Parmenides has conflated mere existence with truth. False beliefs aren't not-being, since they exist (after a fashion), but they also aren't being, since they aren't true. The distinction between the existential form of "is" and the veridical form of "is" and with it the first steps toward modern logic have their roots here. Being is no longer identical to truth, and metaphysics no longer epistemology.
Sophist ends with a proper application of the method of discrimination. They conclude the Sophist is one who multiplies entities endlessly (yum, anachronism!), as opposed to the philosopher, who wields the dialectic to get rid of illusionary entities and false beliefs.