Panzer was the generic short-hand term (in general it means "armor" in German but is also the direct translation of the word "tank") used for a German tank (or tanks) in World War II. The full word is Panzerkampfwagen, which means "armored combat vehicle".
There were multiple Panzers designed and built for the Germans over the course of war, from the lightly-armed Panzer I (1934) to the massive King Tiger Tank in 1944.
The most common German tank throughout the war was the Panzer IV, which was produced in at least eight major variants and many sub-models. However, the best-known Panzers of the war were the Panther medium tank, originally called the "Panzer V" and Tiger I heavy tank, originally called the "Panzer VI Ausführung E". The Panther is considered by many to be the best overall tank of World War II, rivaled only by the Soviet T-34/85. The Panther was also used as the basis for many Western tank designs in the immediate post-war era.
The picture below is that of a Panzer II, which featured prominently in the invasion of Poland in 1939 and France in 1940.