France did a great deal to help the American colonists during their war for independence from the United Kingdom. It is possible to argue that the American colonists would not have won the war if it had not been for the French assistance.
The French did not get involved in the American Revolution right away. The French did want to support the Americans because they wanted to hurt Great Britain (which was their main enemy in Europe). However, they did not believe that the colonists would be able to defeat the British. Therefore, they thought that getting involved would simply lose money and soldiers with no prospect of any return. However, when the Americans won the Battle of Saratoga, France decided that the colonists had a good chance to win. This led to active French involvement in the war.
The French helped the American colonists in two main ways. First, they provided the colonists with many of the supplies they needed and with a great deal of money. The colonies needed these things very much in order to keep their war effort going. Second, the French provided important military assistance. The French sent some ground troops, though they did not send a very large number and those troops were not of tremendous importance. More importantly, France sent naval forces to help fight the British navy. These naval forces were very important because they happened to be present to fight the British at the Battle of the Chesapeake in 1781. The French victory in this battle prevented the British from relieving their besieged troops at Yorktown, VA. When the Americans (with some French ground forces) defeated the British at Yorktown, it marked the end of major fighting in the war.
Thus, the alliance with France helped the colonies win the war by providing them with supplies, money, and military aid.