A Camera Obscura is basically a dark room with a tiny hole in one of the walls, an inverted image of the objects that are outside is formed on the wall opposite of the small hole. Depending on the size of the hole the image can either be sharp and dim. (small Hole) If the Hole is large the image is brighter but more blurry. (Large Hole)

 

I Made some attempts at this photography style with a variety of results, due to a change in size of the hole