The American built M4 Sherman medium tank was one of the most famous tanks of WW2.The Sherman was designed early in the war when the German's had very few tank's that could challenge it.However,by the time it into combat in late 1942 the German's had new weapons,such as Tiger I heavy tank and an improved version of the Panzer mark IV.Despite this the Sherman became the most popular tank in the American army.
The Sherman was given to many other allied countries such as Great Britain and Russia and soon became popular with those countries as well.As the war went on the Sherman was modified more and more.Eventually the Sherman was fitted with a new more powerful gun which helped it fight the bigger German tanks.The Sherman saw service in every major combat theatre(front)of WW2 and served with many armies even after WW2.

A modified version of the Sherman with the better gun.
http://en.wikipedia.org/wiki/M4_Sherman
That's all for now!
Tigertank