Lighting is used to create the mood and style you desire. To create these different types of lighting you can work in the studio using different lighting equipment, or work with ambient and rely on the weather, and other equipment such as reflectors.
Hard lighting is when the light is very harsh on the subject, usually created by a very small light source (in relation to the object). The shadows created by the light are also very harsh. The distance between the subject and the light can have a big difference on the hardness of the light, so the closer the light is to the subject the harsher it is.
A typical soft light source is about the same size as the object. With soft lighting the shadow is a lot softer, there’s more of a gradient from light to dark. The distance between the subject and the light also effects this type of lighting, the further away the light source the softer it is. You can also use a soft box to create a soft lighting effect.
You get diffused light from very large light source. This means that everything behind the object gets some light. The shadows disappear.