I'll give a try at explaining:
SA/DA: in Single Action SA the slide is racked back cocking hammer and then forward so a shell is carried into chamber, then safety may be applied or trigger may be squeezed to fire.
In Double Action DA the slide is racked and a shell is inserted into chamber but hammer is then let down, either manually or with decocker. Then the action of squeezing trigger also cocks the hammer back before firing, of course putting more resistance on trigger pull.
A decocker is a mechanism that once slide is cycled and hammer is locked back in firing position, a lever is pushed which causes gun to go through the motions of firing, as dropping the hammer, but blocking the firing pin so the bullet doesn't fire. Some like myself don't trust a decocker because mechanical failures can and do occur. Others are quite comfortable with a decocker, but are still very careful to always have muzzle [forward end of barrel where bullet exits] pointed in a safe direction. A truly safe direction is not always available, say on a concrete or blacktop surface, something to be considered.
The lightest and easiest triggers are in Single Action. Somebody please correct me if I got stuff a little mixed up?