The act is generally known as lend-lease in the US but lease-lend in the UK. In fact neither term appears in the true title of the act, which is "Further to promote the defense of the United States, and for other purposes.". See  for the text of the act. More information, and images of the text, from .
Lend-lease was a critical factor in the eventual success of the Allies in World War II, particularly in the early years when the United States was not directly involved and the entire burden of the fighting fell on other nations, notably the British Commonwealth. Although Pearl Harbor brought the US into the war in December 1941, the task of recruiting, training, and equipping US forces, and then transporting them to the war zones could not be completed overnight: through 1942, and to a lesser extent 1943, the other Allies continued to be responsible for most of the fighting, and the supply of military equipment under lend-lease was a major part of their success.
Even after the United States forces in Europe and the Pacific began to reach full-strength in 1943-44, lend-lease continued. Most remaining belligerants were largely self-sufficient in front-line equipment (such as tanks and fighter aircraft) by this stage, but lend-lease provided a useful supplement in this category even so, and lend-lease logistical supplies (including trucks, jeeps, landing craft, and above all the Douglas DC-3 transport aircraft were of enormous assistance.