Unless you have a DAO gun (double action only) or a bobbed hammer.
I will go into a bit more detail while we are at it.
A double action will cock the hammer and rotate the cylinder while you are pulling the trigger. The mechanisms are linked together. That way if you have to get off a quick shot you can just draw the pistol and squeeze the trigger, and keep pulling for the next 5 shots. The down side is they have long, often heavy trigger pulls. Not real great for accuracy. A nice quality revolver will have a smooth long pull, a cheaper one may not feel so nice

. I personally cannot stand a double action pull, but that is just me.
A single action (think ruger redhawk or old western style six shooters) requires you to pull the hammer back for every shot. The down side to this is you may have to change your hand position and re sight the weapon for every shot. More time consuming. The upside is they have much better trigger pulls which would help you shoot more accurate.
And most double action revolvers you can shoot in single action. You just pull the hammer before you shoot it. That is providing it has an exposed hammer. I believe some combat models don't have them, all the ones I have seen were smaller framed revolvers though made for concealed carry.
Paul