The morning of April 19, 1775 saw soldiers of the British Army arriving at the Massachusetts town of Lexington. Their mission was to seize and destroy militia weapons and ammunition, but the local militia, known as Minutemen, stood on Lexington Green, awaiting their arrival. During the stand off, a someone fired a shot, which led the British troops to fire at the colonial militia. The Minutemen dispersed, and the British headed toward nearby Concord.
At the Concord North Bridge, a small group of militia battled a force of British soldiers. At this point, the British commander decided to retreat back toward Lexington, as it became evident that more and more Minutemen were arriving from all of the local villages and farms.
During this retreat, the British kept to the road, while the American farmers fired at them from behind trees, walls and any obstacle they could find. When the British force returned to Lexington, they were met by a relief column. The combined British units then headed for Boston. The Minutemen continued to harass them the whole way.
By the end of the day, British casualties numbered 273, while the colonials suffered only 94, 18 of whom fell during the initial clash at Lexington. The American Revolutionary War had begun.
More info from The History Guy.