H2S is a twin sky tuner - like Sky+ or Sky HD. If you don't have Virgin as your internet provider but have a sat dish this is the way to go.
H2H is a sky tuner and a virgin tuner in the same box. If you have VM as your internet provider and a sat dish, the H2H in theory is the way to go.
Other models are older versions; S, 2S, H and so on.