It's an awesome little device that comes bundled with HeartGold/SoulSilver. It's a pedometer based gadget that wirelessly syncs to the game cartridge itself. Basically, first thing you do is transfer a Pokemon from your HG/SS game to the device. Then, you put it on your belt and go about your day. As you walk, you gather 'watts'. Those watts can then be used to access minature areas, each containing four patches of grass, which contain wild Pokemon like in the regular games. The more watts you collect, te better areas you unlock, and the more costly areas have rarer pokemon. You use the Pokemon you stored in the device to battle the wild Pokemon(battles are much more simplified; due to obvious graphical limitations, the mini sprite you see in the 'party menu' represent the Pokemon. Each pokemon has 4 HP, and you have the option to 'Attack', 'Dodge', or 'Catch'. The Pokemon caught on the Pokewalker can then be transferred to the HG/SS games themselves.
Some of the Pokemon on the Pokewalker are going to be otherwise unavailable in HG/SS, so it seems. I think it's going to be awesome.