The basis of a true map campaign is that the players move armies across a map, battling other players as their armies meet. The campaigns are made more interesting by adding rules like orders, settlements, supplies, terrain to name just a few. True map campaigns can be as complicated as you like, just as long as all the players involved are committed to keep a complicated going with all that it entails.
Much like node campaigns, true map campaigns can be quite restricting in regard to who plays who in the campaign as it's completely based on their position on the map. It's quite possible to play the entire campaign and never battle against a player who's forces are on the far side of the map from you. Now it might seem boring to be playing the same few players who border your region but the map allows the games to take on a narrative and so grudges are remembered, alliances are formed and broken as you play through the campaign writing history with every dice roll.
The map that's used for the campaign can coming in various forms from a basic sketch that armies can move freely across, to grid maps where forces move from segment to segment, to the glorious plastic planetary empires maps. It doesn't matter what sort you use, just as long as the rules are clear on how you move your forces across it, whether it's measured in inches or from hex to hex.
Have you played a map campaign? add your thoughts and experiences in the comments below.