[masterofweirdness]

I like to think of a roleplaying game like dark heresy (NOT like a video game... pen and paper RPGs are by far much better, imo) as a sorta story-telling game. Basically, all the PCs (player characters) control the "heroes" of the story, while the GM (game master) controls all the other characters, from allies, enemies, merchants and minor characters.

Now, the reason you need a rule book to play RPGs is for two things: setting and rules. Dark Heresy gives you all sorts of information on the setting of Dark Heresy, the Calaxis Sector, and how the Inquistion works in this sector. It also provides you with rules, rules for all the guns and swords your PCs will use, how to pick locks, smash open doors, and kill people, etc.

If you think that sounds like fun... pick up the book, it is truly an amazing game.