Your primary strategy for placing money into the pot is to either bet or raise for value or to bluff. These two reasons predominate throughout poker and is the first and probably the biggest leak that players need to fix at the lower levels. However our money at certain levels of play has to come from somewhere and it is either through bluffing or value. Now before we continue I am not saying that you can only make money by bluffing or by value betting at any stake level. What I am saying is that there will be a strong bias towards one or the other.
Let us take the smallest possible stake level in online poker which is $0.01-$0.02 cash games. Bluffing in a game like this will be far less effective for the simple reason being that your opponents will simply call more. The combination of the small stake levels and the low sophistication of your opponents mean that these guys want to play hands and not to fold them. So your primary reason for betting (notice that I said primary and not solitary) is for value and not to bluff.
However let us not change that to say a NL50 full ring cash game with $0.25-$0.50 blinds. This is still not high stakes by any stretch of the imagination but $50 buy-ins represents a very credible stake level. So there will be a lot of tight and solid serious players at these levels as well as several minimum stack players. So value betting through the streets becomes less effective for several reasons. The first is because your opponents are tight enough not to call with weak second best hands. You will be strong enough to be raising and re-raising to isolate and so you will not extract much by way of value from weaker holdings most of the time. Secondly the presence of the minimum stacks reduces your value betting potential.
This is not to say that it will never happen, just that your bias needs to be leaning towards bluffing and extracting dead money from your opponents. Of course there will be times where you can value bet but those times are reduced at this level because many of the pots will be heads up and three way. Your tight mostly multi-tabling and short stacked opponents will be biased towards folding. The mentality will be different in a much more aggressive six handed game but in full ring then we need to reassess the situation.
However at say the $10-$20 levels then your opponents are not going to call you down loosely unless they have a reason to not believe you based on your image. Although in a strange kind of way you can value bet these higher levels more than you can at say NL50. This is because the average player is more sophisticated and is more than likely properly bankrolled for their level. This means that they are prepared to call you down very thin if they think that your range is polarised or because of your recent image.
So the bottom line is that you need to be aware of the two primary reasons for betting and then see which one is more relevant to the particular hand in question. You also need to build your entire strategy around these reasons as well. Many players fail to beat levels like NL25-NL100 because they expect too much value from their good hands. In actual fact if only these players shifted their reasons for betting away from value betting to bluffing then they would do far better. At each level there tends to be a theoretically correct default setting for the proper levels of value betting and bluffing.