Who they were
Cleisthenes was an Athenian aristocrat of the Alcmaeonid family, active in the late 6th century BC. His birth and death dates are unknown — a birth around c. 570 BC is conventional — and he disappears from the record after his reforms. He is often called the father of Athenian democracy.
What they did
After the fall of the Peisistratid tyranny in 510 BC, Cleisthenes faced a power struggle with Isagoras, who called in a Spartan king; a popular uprising restored Cleisthenes, and he carried his reforms in 508/507 BC. He created ten new tribes that deliberately mixed city, coastal, and inland demes, breaking the regional blocs of the aristocracy, and made the deme the unit of citizenship. A Council of Five Hundred — fifty per tribe — was established, and the assembly became the decision-making center. His own watchword was isonomia, equality before the law. Ostracism, the ten-year exile decided by vote, is attributed to his reforms per the Aristotelian Constitution of the Athenians, though its first known use came only in 487 BC.
Legacy
The machinery Cleisthenes built ran Athenian democracy for nearly two centuries. It remains the classic case study of institutional design against faction.