Definition of value bet
A value bet is when a player bets with one hand and hopes to be called by a worse hand. A value bet is when a player has a good hand to call. Sometimes a player can bet with a good hand and get called with a better hand. In this case, the player is said to have "value-owned", i.e. he bet believing he was going to get money from a worse hand, but in the end, simply lost more money on the worst hand of the two.
A value bet is the opposite of a bluff.
A value bet is the opposite of a bluff. When you bluff, you have a weak hand and you hope your opponent will fold. When you value bet, you have a good hand and you hope your opponent calls with worse. The size of the value bet should always be evaluated based on the strength of the hand that we suspect our opponent has rather than based on the strength of our own hand. Even with a very strong hand, if you think your opponent is weak, you should bet small to encourage your opponent to call with his worst hand that could not afford to call a big bet.