Date: September - October 1781
In the summer of 1781, The British General, Lord Cornwallis, was ordered to secure a deep water port in Virginia. He'd chosen Yorktown, on the Chesapeake Bay and by the end of September his troops had fortified the city and were awaiting resupply from ships in New York City. (What Cornwallis didn't know was that the ships he was expecting for resupply had been defeated by the French navy at the Battle of the Chesapeake in early September. He could have probably done knowing that.)
On the rebel side, George Washington had just secured the help of the French, and was anxious to put his new troops in the field. He favoured an attack on the British stronghold at New York - but, since he didn't have the troops or equipment for that, he settled for attacking Cornwallis at Yorktown (I'm sure Cornwallis would have been flattered by the news he was second choice).
The main force of Continental and French troops marched to Yorktown near the end of September and began to settle in. By October 6th the trenches were built and the siege had begun.
The key to Washington's strategy was the artillery brought in by the French army. It bombarded the city for more than a week, wearing down defenses until French and Continental troops successfully stormed two redoubts outside the city on October 14. This put the artillery in a place where it could reach the city itself, and Cornwallis' defenses quickly crumbled under the onslaught. He surrendered on October 19th.
This fall of Yorktown essentially broke the political will in Britain to continue the war - it had become too costly. Parliament started peace negotiations, and British citizens began to stamp their tiny feet and practice their speeches about how the colonies were smelly and uncivilized and we didn't want to control them ANYWAY.
Yeah, stinky colonies!