Gruppenfuhrer was a Nazi rank roughly equal to major general. In English, it translates to "group leader", which, again, was roughly the role of a division commander. It was primarily associated with the SS, although the SA did use it.
Ranks above it were Obergruppenfuhrer and Oberstgruppenfuhrer. Reichsfuhrer was reserved for Heinrich Himmler and does not really count in the rank system; Himmler was senior to all SS personnel. The rank below was Brigadefuhrer.