Poker is a game of incomplete information.  Even the best players can only guess what their opponents hold most of the time.  For years most poker analysis has assumed perfect, complete knowledge of the opponents holdings.  While it might be interesting to ask: "what are the odds that A♠A♣ will win versus K♥4♥," it isn't a question that you will likely ever ask while actually playing poker.

In order to arrive at results which are useful to making sound poker decisions we need to rephrase the problem in terms of the information that we might possibly have at the table.  Usually, we know what specific cards we "the hero" hold.  But the best we can do is put our opponents on a range of hands.  Even if we know that a player only raises with aces, that fact represents 6 distinct hands.  Since it's impossible to put a player on two black aces, the best we can do is calculate our equity versus all six different match-ups.

PokerStove is a poker utility which facilitates equity calculation using ranges of hands, or hand distributions.

PokerStove