Short answers, pulled from the story.
Brennus was a Gaulish chieftain who led the Senones, a tribe originating from the region of present-day France around Seine-et-Marne, Loiret, and Yonne. A branch of the Senones had crossed the Alps around 400 BC and settled on Italy's east coast, founding Sena Gallica (modern Senigallia) as their capital.
Brennus defeated the Romans at the Battle of the Allia around 387 BC, then led his forces into Rome and occupied most of the city for several months. The Senones held Rome until being expelled, according to one tradition, by the exiled Roman dictator Marcus Furius Camillus.
According to Livy, Brennus threw his sword onto the scales during a dispute over the weights used to measure Rome's ransom of one thousand pounds of gold, and uttered "Vae victis," meaning "woe to the conquered." The Gauls had brought heavier-than-standard weights to the negotiation.
According to Plutarch, Camillus returned from exile, drove the Gauls out of Rome's streets, and then annihilated them in a pitched battle eight miles outside the city on the road to Gabii. His troops hailed him as a second Romulus and gave him the title Pater Patriae, second founder of Rome.
Brennus's capture of Rome around 387 BC was the only time in approximately 800 years that a non-Roman army occupied the city, a record that stood until the Visigoths took Rome in 410 AD. The Capitoline Hill was the sole part of the city to hold out against the Senones.
Geoffrey of Monmouth included Brennus under the name Brennius in his Historia Regum Britanniae, and the Jacobean play Fuimus Troes featured his ghost supporting Britons against Julius Caesar. Paul Jamin painted Brennus and His Share of the Spoils in 1893, and Gordon Mitchell portrayed him in the 1963 film Brennus, Enemy of Rome.