A brief orientation
Pericles was an Athenian aristocrat (his mother's family, the Alcmaeonidae, were prominent for generations before him) whose career carried him from his first political action as a young man through three decades of nearly continuous influence over Athenian politics. He held the strategia — the elected generalship — fifteen times. Under his political direction Athens rebuilt the temples of the Acropolis after the Persian destruction, consolidated the Delian League into the Athenian arche (empire), and entered the long Peloponnesian War against Sparta. He died of the plague in 429 BCE in the second year of the war.
The sources
The principal contemporary source is Thucydides, who knew him, served as an Athenian general in the war Pericles started, and gave us — in the Funeral Oration of History Book II — the most influential single ancient statement of the Athenian self-conception under his leadership. Plutarch's Life is the long biographical treatment. Plato's Gorgias and Menexenus engage with him critically.
What the Periclean Athens was
The Athens Pericles led was a democracy with an empire — an unusual combination of forms. The Athenian demos met every nine days in assembly and decided foreign and domestic policy by direct vote; the empire that produced the revenue paid the rowers of the fleet and built the temples. Thucydides' famous summary judgement on Pericles — that the city was nominally a democracy but in fact ruled by its first citizen — is the locus classicus of the question of how a republic can be effectively led without being thereby unmade as a republic.
Why he matters for Virtue & Power
Pericles is the platform's case for the statesman who leads a democracy through extended sustained policy without seizing it. The contrast with the late Roman Republic is direct: Pericles' Athens did not need a Caesar because Pericles held office under the ordinary institutions and kept them. That this was harder to sustain after his death is part of the long Greek argument the platform's later entries take up.