Thor was the German god of strength, and the god of thunder and lightning (like Jupiter or Zeus). These three gods of the Greeks, Romans, and Germans were more or less the same because they all came from the same older Indo-European god. He carried a big hammer. Because he was so strong, the Germans sometimes thought of Thor as rather stupid, but it was his strength that defended the gods and people against the dangerous giants in German myths.
Here's a video about a fight between Thor and Loki: