Short answers, pulled from the story.
Gaius Marius was born in Cereatae, a small village near the town of Arpinum in south-east Latium. He was born into a locally important family of equestrian status with resources large enough to support two members of the family in Roman politics.
Marius attained his first consulship in 107 BC and became the commander of Roman forces in Numidia where he brought an end to the Jugurthine War. He subsequently served as consul every year from 104 BC to 100 BC while defeating the Teutones at Aquae Sextiae and the Cimbri at Vercellae.
Scholarship starting from the 1840s attributes broad reforms to Gaius Marius including the recruitment of volunteers from the poorest census class known as proletarii for the war against Jugurtha in 107 BC. Other claimed reforms include the abolition of citizen cavalry and light infantry, a redesign of the pilum, a standardized eagle standard, and the substitution of the cohort for the maniple.
Gaius Marius died within a fortnight of assuming the consulship for the seventh time after participating in a purge of opponents following the civil war of 87 BC. His death occurred shortly after he returned from exile in Africa to Etruria with an army raised there to place himself under Cinna's command.
Gaius Marius is considered responsible for the beginning of the Roman revolution because his unquenchable ambitio overcame judgment and led to disastrous civil war in 87 BC. His use of the Assemblies to remove Metellus from command in Numidia spelled an end to collective governance in foreign affairs and challenged traditional republican culture based on short terms of office holding.