Socrates
What the sophists were
The sophistai were a recognisable group of itinerant teachers active in fifth-century Athens and the wider Greek world: Protagoras of Abdera, Gorgias of Leontini, Hippias of Elis, Prodicus of Ceos, Thrasymachus of Chalcedon, and others. They taught for fees, often travelled from city to city, and offered training in rhetoric and in the kind of public competence that was useful in the democratic Athenian setting. The word itself was originally not pejorative; it acquired its later pejorative force largely through the Platonic dialogues.
How Plato sets the contrast
Plato puts each of these figures into dialogue with Socrates and treats the distinction between them as serious. Socrates does not travel as a paid teacher; he does not promise to make his interlocutors successful in the assembly or in the courts; he does not present his work as a technē one can buy. He argues with the sophists about whether virtue can be taught at all, whether appearance is the same as reality, and whether justice is anything more than the interest of the stronger.
The contrast is sharpest in the Protagoras (whether virtue is teachable), the Gorgias (rhetoric and the good), the Republic (Thrasymachus' opening definition of justice), and the Sophist.
A note on the historical sophists
Plato writes with philosophical purposes of his own, and modern scholarship is careful not to accept his portraits of individual sophists as straightforward biographical history. The historical Protagoras and Gorgias are recognisable but reconstructed figures, not the same things as their portraits in the dialogues. We treat the Platonic Socrates–sophist contrast as Plato's contrast, and we do not pretend it is a transparent historical record.
Why it matters for Virtue & Power
The Socrates–sophist contrast is the founding instance of a question the platform returns to often: what distinguishes the teaching of genuine excellence from the teaching of skills that look like excellence but are not. The classical version of that question is worth reading on its own terms before it is mapped onto any modern analogue.