Human steam-powered ironclads versus Martian windships: the basic rules are straight-forward and map out hex-based movement and combat. Once you learn these, there are rules for boaridng parties, designing new ships, new weapons, etc. The manual gives a lot of theme and steampunk flavour. The book also supplies ship damage forms, and pics of the original hex maps. If you buy this book, you will need some dice, a big hex map, and something to use as counters for the ships.
I bought an original box set off eBay. Although the maps look great, the cheap plastic ship minis are chintzy and ugly. You'll want to make up some nice counters - or better yet, buy some sterampunk-looking minis for Aeronef or Dystopian Wars.
I recommend Googling 'Cloudships of Mars': some fans have made some short-form torunament rules and some updated ship damage forms that work great!