Guests in the forums are generally members who have asked for their account to be deleted. That is, if you're referring to when it displays 'guest' under a users nick name, instead of Member.
Even if you ask for your account to be removed it DOES NOT remove any forum posts you have made, otherwise it disrupts the flow of threads to have random posts removed throughout.
Rep power is a complicated one, that not even I understand full, but it is given when someone adds to your rep. It doesn't really do or mean anything. The 'like' button is more commonly used and easier to see. Most people use it instead. It was added to the forums after the Rep power was.