Hard light, Soft light
This one should probably be an elemental and simple concept for many people, but others, specially the ones who are starting to shot with artificial lighting, are confused about it.
Light, whose rays are hitting the subject from the same angle, produces a high contrast situation, with neat and well defined shadows, HARD shadows, from which it is called hard light.
When instead the rays are hitting the subject from different angles they produce a low contrast situation, with unsharp shadows, SOFT shadows, so it's called soft light.
If there's just a single light source there are two factors that are determine which kind of light it produces: its size compared to the subject and its distance always from the subject.
The smaller the light source is compared to the subject the harder the light is, the farteher is the light source is from the subject the harder the light is, and, of course, the opposite.
These concepts are verifiable by anyone without any camera or other instrument. Just think at a sunny day. The Sun, looked from Earth, is a small and incredibly luminous sphere, in reality it's very large but also very very far from us. As its distance from us is very huge and we just see a little sphere, the rays of its light are hitting us from the same angle. So in a sunny day all the objects have a sharp and neat shadow. It just takes to have some clouds in the sky before the Sun that the light source is not the sun anymore, but the clouds that are before it. And the clouds are a lot nearer to us then the sun so they seem also a lot larger. And we could verify then that the shadows are soft and unsharp.
The kind of light influences the shadows and the reflexes (hard light = sharp shadows and brilliant reflexes, soft light = unsharp shadows and dull reflexes) but not how dark the shadow is. This is determined by the amount of light hitting the area in shadow. For this reason, in photography, to brighten up (gergal term = open) the shadows we use reflectors, alas instruments that reflect the light in the areas where there's too few of it.
Esempi - Examples
Hard light, look at the sharp contour of the shadow and the sparkling reflexes on the apple.
Hard light with a reflector on the opposite side, the shadow is still sharp buit a lot less dark, there are still the sparkling reflexes on the apple but its shadowy side is almost as luminous that the side facing the light.
Soft light, the shadow is unsharp while the reflexes on the apple are dull.
Soft light with a reflector on the opposite side, the shadow is almost gone, the reflexes on the apple remain dull but the apple looks evenly illuminated.
Conclusioni - Conclusions
What you have to do now is testing, testing and again testing with the various kind of lights, mixing them, adding reflectors, etc... As you could have seen there's not an universal recipe, it all depends on what you like and which kind of image you want to create.
The important thing is that YOU have to be in control of the light and not the opposite.