Hard lighting can be used for making a subject appear like it stands out more than usual because it is outlined by a hard shadow. Lighth waves travle straight; they do not bend therefore they cause a sharp shadow behind the object. Hard light may be achieved by taking a photo in the sun with out a flash or it may be taken in a theater to get a more picturesque image of the stage production.
Soft lighting, where there is no defanite shadow and everything has a soft edge can be achieved by taking your picture under artificial lighting with a flash. You would use soft lighting for taking images for a catologue or portraites of people. Also you may want to use soft lighting when taking a macro shot of some flowers.
The mood of a picture may be changed with lighting because the lighting causes you to precive the image differently and it sets a mood that you are familiar with.