A value bet is a bet that, when called, wins more than half the time. The test is not whether your hand is strong but whether, of the hands that call, you beat most of them.
On the river there are no more cards to come, so a bet has exactly two jobs: get called by worse, or make better fold. A value bet is the first job. Top pair with a good kicker on a dry runout usually passes the test, because villain’s calling range is weaker pairs and ace-high that refuses to fold. Middle pair usually fails: the hands that call have it beaten, and the hands it beats fold. A bet with a hand that only better hands call is not a value bet; it is a bluff you cannot win with.
Strong hands should start betting early. Pots grow geometrically — bet two-thirds of the pot on the flop, turn and river, and a pot of 8 grows to about 100, more than twelve times the start; skip a street and it ends near 44. So sets, two pair and overpairs on safe boards bet the flop and plan three streets. Size the bet to what worse hands will call: a polarized river range can bet big, while a thin value bet uses a small size that a worse pair can still pay.
Worked example
FAQ
How do you know if a bet is a value bet?
List the hands that would call. If you beat more than half of them, it is a value bet. If only better hands call, check: the bet loses money every time it is called and gains nothing when worse folds.
How big should a value bet be?
As big as the worse hands you want to get called by will pay. Polarized ranges with the nuts can bet big or overbet; medium-strength value hands use a third to a half of the pot.