hi there. sorry abt the brief notes. ;p
actually there are few methods to get this effect,
like what explorerZ has mentioned,
you can just adjust the opacity to get the effect, but that will apply to the whole picture.
normally what i did is localize adjustment,
hence I use eraser tool instead of adjusting the opacity for the layer,
i adjust eraser tool opacity.
so u can erase 25%, 50% and 100% opacity wherever you like.
hope this help.