Units are individual AFV's, light weapon teams and infantry sections and the basic game covers a selection of theatres including the Western Desert (above), Normandy and the Ardennes. There is extra downloadable content covering other theatres such as 1940, Sicily etc. (which you pay for) and also quite a bit of free stuff designed by fans of the game which includes the Pacific, Crete and WWI. I actually started with the WWI stuff and must say the Cambrai campaign was superb and extremely challenging, especially the canal bridges battle (below).
All in all, if you have an iPad and are interested in WWII (or WWI) this should be a must have game.