From the back of the book:
Battlegroup is a set of tabletop wargame rules for recreating the battles of World War II, with 20mm or 15mm miniatures.
This is a core rulebook, detailing how to play the game, including such basic rules as Command and Control, Movement, Firing and Morale.
Command and Control: The rules use a system of Orders, putting difficult tactical decision making in the centre of the game.
Firing: A unique 'two-tier' system for firing involves Area fire and Aimed Fire, in which pinning enemy units can be as important as destroying them.
Morale: The innovative Battle Rating counters system means no two game will ever play the same.
To play you will also need a collection of wargames miniatures, a wargames table, an opponent and one of the Battlegroup Theatre supplements, which provide all the detail, army lists and character of specific battlefields and periods of war.