Poker Wiki

Strictly speaking, a hand range is the total 'range' of hands which your opponent may hold at any point in a hand, or in a given situation.

When the hand begins, your opponents have a completely random hand (100% of all possible starting hands. A very nice tool for doing hand range analysis is called Poker Stove.

If you have statistical data on the players at your table, such as the overall percentage of hands they play, the percentage of hands they raise, etc, you can narrow the possible starting hands they have if they enter a pot. Thus you can narrow their hand range.

Now if you have more evidence of how your opponents are likely to play a draw, or a made hand, you can further deduce what their hand range may be on further streets, based upon how they acted before the flop, on the flop, turn etc.