Set in the three most important days of the battle for Gettysburg, this huge game is extremely detailed. As well as the normal terrain features you'd expect (clear, woods, hills, rivers, roads), the map features different kinds of roads, varying elevations, differing slopes on the hex edges, and differing levels of human habitation (buildings that might get in the way).
There are several different kinds of units - not just infantry, cavalry, and artillery, but different types of them. About 8 different kinds of artillery and infantry, with varying levels of performance. Units have a morale rating, which determines how much fire they can face before likely becoming demoralized, and some units are green. Units have varying amounts of troops in them, and the scale is small enough that some units can be deployed to take up 2 hexes when marching or in line formation. (One unit is big enough to take up THREE hexes when fully deployed!)
The level of detail is at first staggering, but once you've invested the time to learn the rules, the game flows by very rapidly. However, this is NOT a quick game! This is something you devote a weekend or two to, and a big table.