Wolf Lords are basically the Space Furries counterpart to a Chapter Master. The Wolf Lords are the leaders of the twelve Great Companies of the Space Wolves, a rank equivalent to a Captain in other Chapters.
Unlike other Space Marine Chapters, which rigorously follow the organizational dictates of the Codex Astartes, each of the Great Companies takes its name from its current Wolf Lord, and often changes subtly in character with each new Lord's approach to organization and tactics. For instance, Bjorn Stormwolf's Great Company has become famous for leading planetary invasions with as much fanfare, noise, and firepower as possible, a clear reflection of its Lord's temperament. A Wolf Lord may be accompanied by the huge Fenrisian Wolves and always by a small coterie of bodyguards and advisors specifically loyal to him, known as the Wolf Guard, whose job it is to choose the Lord's successor when he dies. This successor is often (though not always) chosen from the ranks of the Wolf Guard itself.
When he is elevated to his position, a new Wolf Lord chooses a symbol or badge from the ancient heraldry of Fenris; these symbols are often re-used, since Lords seek to inspire their Companies to live up to the reputation of their sigil's previous users. The chosen wolf totem of the Wolf Lord adorns his Great Company's banner and is painted on the shoulder plate of every Space Wolves warrior in the Great Company that he commands.
The badges of the Great Companies are displayed on the Grand Annulus, a circular disk made up of triangular slabs of stone on the floor of the Hall of the Great Wolf inside The Fang. Each slab carries the badge of a Great Company, while the central slab has a pointer, indicating which of these Companies belongs to the current Great Wolf.
Essentially a Wolf Lord on steroids....or....a Chapter Master....of Chapter Masters. The Great Wolf is the overall leader of the entire Space Wolves chapter and high-king of Fenris, chosen from the Wolf Lords. The current Great Wolf is Logan Grimnar who has held the position for the past 600 years. Whilst regular Wolf Lords are leaders of one of the Great Companies, the Wolf Lord of the leading Great Company is known as the Great Wolf.
Tl;dr, they are the Alpha Wolf. Pun intended.
|Forces of the Space Wolves|
|Command:||Wolf Lord - Wolf Guard|
|Priesthood:||Iron Priest - Rune Priest - Wolf Priest|
|Troops:|| Blood Claw - Fenrisian Wolf - Long Fang - Skyclaw |
Swiftclaw - Thunderwolf Cavalry - Wulfen - Grey Hunter
|Vehicles:|| Bike Squad - Rhino - Razorback - Land Speeder |
Predator - Vindicator - Whirlwind - Space Wolves Dreadnought
Land Raider (Land Raider Crusader
Land Raider Redeemer - Wrath of Mjalnar)
|Flyers:||Stormfang - Stormwolf|
|Allies:||Space Marines - Primaris Marines|