Ground units are corps, steps are divisions. Air units are wings, each step is 100 planes. Air units can react two hexes after the bad guys move. Combat is between units in the same hex. Units throw dice equal to their steps in combat, but hit on different numbers depending on their ratings for ground, air, or naval combat.
Units also have different movement rates.
For example, an infantry corps has an air value of 1, a naval value of 0, a ground value of 2, and moves two hexes per turn. An armor corps has the same combat values, but moves three hexes per turn. Another important distinction is that firing is sequential, and some unit types get to fire before others. Thankfully, unlike Blitzkrieg, Victory seems to be a fluid game that feels something like WW2. A promised expansion set should add artillery, mech, cav, mountain, shore batteries, cruisers, flak, torpedo bombers, and mobile supply units to the game.