Similar to Axis & Allies, you can build infantry, tanks, SAMs (surface-to-air units useful against aircraft) rather than AA guns, fighter jets rather than planes, bombers, subs, transports, aircraft carriers, destroyers, and factories. I think the game would benefit by having a battleship as the king of the sea, and maybe a missile cruiser as intermediary between subs and destroyers to offer a little more variation for sea battles, but the naval options available work pretty well. Aircraft carriers allow fighter jet to land on them (but not bombers), so that they can serve as islands from which to launch fighters against multiple targets. Don't lose your carrier though, as the fighters will crash in the sea unless they are close enough to a friendly country to land!
There is nothing to download, as the game can be played completely in your browser (although I think there is an option to download if you want to play while off-line). Click the link below to check out the site, but beware; its addicting!