The Jacobins were members of the Jacobin Club, the most radical political society to rule during the French Revolution. The society got its name from its Paris headquarters in the street of St. James (Jacques in French). It was the only national organization in the country for a while after the Revolution began. The Jacobins came mainly from the middle class.
They opposed foreign war at first, fearing that it would lead to a military dictatorship. But when war broke out with Prussia and Austria in 1792, the Jacobins supported it in order to gain control.
The Jacobins came to power in 1793 and began the Reign of Terror, sending hundreds of Frenchmen to the guillotine. Robespierre was the most influential Jacobin leader. But his fellow Jacobins turned on him in 1794 and executed him. After his death, the Jacobins lost power. The term Jacobin is still used to describe a radical.