The river
The river will have you faced with the last decision you’ll make on any particular poker hand. As such, it’ll all come down to losing as little as possible on the worst hand and making as much money as possible when in possession of the best hand.
Even though there is a certain intricacy to correct river-play, you should always aim to have things clear by the time you reach that stage: that is, weak hands should not be taken to the river, and any hand worth taking to the river, should be as good as to be treated as the best hand with confidence.
Most often, you’ll get involved in sparring on rags on the river when you chase after flush and miss. Chasing 4-card flushes can be the correct thing to do under certain circumstances, thus, you may end up having to deal with the river on a hand you’d never normally have taken that far.
If you do end up in that situation, there are a number of decisions you’ll have to deal with: Do you raise, or do you call? If you know your opponent might hold a marginally stronger hand (a low pair against your high-card) in that situation, it pays to bet. If you succeed to make your opponent fold, you’ve played it well.
The same thing though, does not work vice-versa. If you’re holding the marginally stronger hand, you do not want to force your opponent to do the right thing and fold.
Calling against your opponent’s raise on rags, is – needless to say, I hope – not a good course of action. In that case, you’re better off folding.
If you have a monster, but you know your opponent also has something big, (you know he’s made a flush, but you have an A-high one) you should definitely get him all-in. In case you played your cards well on the flop, that shouldn’t be difficult at all.