Nobackwheel:
If you're simply wanting to hang something like a calendar, then a nail is fine, or even an adhesive hook.
If you're wanting to mount something like a wall clock, which will break if it drops, then a hollow wall anchor, or even a plastic anchor, is all you need.
If, however, you want to hang a heavy picture, then you'd do better by going out and buying a tool called a "Zircon Stud Sensor" to locate exactly where the studs are in this wall, and then driving a screw through the plasterboard and into the wooden stud. Then, hang the picture on the screw. Alternatively, for a bigger picture, put screws in at the same elevation on two neighboring studs, and hang the heavy picture from both screws.
I'd prolly use 1 3/4 inch #8 or #10 sheet metal screws, depending on how heavy the picture was. (The only difference between a wood screw and a sheet metal screw is that the sheet metal screw is threaded all the way up to it's head, whereas the wood screw will have an unthreaded shank before the head.) Predrill to the correct depth with a 7/64" drill bit for a #8 screw and a 1/8 inch drill bit for a #10 screw.
http://www.zircon.com/sellpages/ScanAndSensor/SS/SS.html
PS: Hollow wall anchors have a threaded end on them. You drill a hole through the plasterboard, insert the hollow wall anchor into the hole, and then tighten the screw. As the screw tightens, the legs of the hollow wall anchor spread out behind the plasterboard, locking the anchor in place. Once the anchor is locked in place, you can remove the screw and put it back in again. Not sure if this explains things better.