Divisions are the basic level of organization in Steel Division (shocking, isn't it?). Each division can be one of four types, with their own units to choose from, advantages, and disadvantages. Each division also has its own limit of activation points and number of slots to fill in while creating your battlegroups.
Infantry divisions rely heavily on cheap and numerous infantry, infantry tanks & artillery. Less mobile than more mechanised ones, they can slowly grind an enemy to death.
Mechanised divisions are 'reinforced' infantry divisions, with armoured transport, assault-guns and/or self-propelled artillery & anti-tank guns.
Armored divisions usually rely on tanks, some heavy infantry in armored transport and self-propelled howitzer & anti-tank guns. They are the most mobile divisions.
Nemesis is a post-DLC concept for Steel Division 2, which pits two divisions - one Allied, one Axis - against each other, inspired by some of the notable battles of the European theater of war in 1944.