You have been betting all the way with a reasonably strong hand like top pair with an ace or king kicker! You get to the river and the board still looks reasonably innocuous. You bet on the end for value and all of a sudden you cop an unexpected raise. What should you do?
If you read a book like "Hold'em for Advanced Players" by David Sklanski and Mason Malmuth they suggest that you very rarely win when you are raised on the end, but suggest you should still call if the pot is big enough. However, in fairness, I think the games that Mason and David typically play in, at the time of writing their book, are not the sort you will find poker online. The action is generally much wilder online. I have found that a raise on the end when they board looks innocuous is usually a raise/bluff. This only holds if your opponent has not raised in any other betting round and the river card looks so mundane that you are reasonably sure it has not improved anyone. Think about it! Why would your opponent raise on the river with a strong hand, when the more likely, and correct, place to raise is on the turn. If the river card does not seem to have helped them then it is a good bet that their raise is a bluff or they are raising with second pair hoping to induce a fold. So what should you do?
If I have a reasonably good read on my opponent, and I have seen them try this sort of thing before, then I re-raise. However, I don't think there is generally all that much value in re-raising because your opponent is unlikely to call you re-raise if they are bluffing. You might occassionally capture an extra bet by re-raising if your opponent is bluffing with second pair, but this is offet by the fact that your opponent ocassionally will have a strong hand and they cap it. I think the play is just to call in and be satisfied that your opponent was silly enough to try to bluff you out. I would never fold to a single raise on the end given the above circumstance!
You hold KQo and are heads up, the flop comes:
You bet the flop and get called, the turn comes:
You bet the turn and get called, the river comes:
You bet and get raised.
The above scenario looks to me like a raise bluff. This sort of thing happens a lot at medium to low limit Hold'em online. Your opponent was most likely on a flush draw after the flop and decided that they would bluff the river if they missed their flush. My advice is to just call here, because there is not much value in raising and no sense in folding.