The Battle of Stalingrad was a major turning point because Hitler was hoping to surprise the Russians and take Moscow. When the battle turned into an urban warfare zone, house to house fighting, it caused the attack to slow to a halt. Germany couldn't advanced. Russia was throwing men, they had the population numbers, at the German army. Russians had to fight. They could fight and die against the Germans, or they could attempt to retreat and be shot by their own Russian officers.
Enemy at the Gates is a good movie to give you a visual of what the fighting looked like.
