This photo labels each lights intensities well through making the ones needing less power more feint. Back lighting means that the Key and Fill lights shadow will be eliminated.
Key lighting is the Brightest lighting that is meant to be aimed at the focus of the shot.
Fill lighting is to make the other side of the focused person or item slightly lit, so it isn't completely dark.
The Key light should not be placed directly in front of the person in focus as then it gives a 'deer in the headlights' look which is not very flattering. It should be at an angle so that features of the person in focus are seen in more detail with more defined looks. It can also cover one side of the person in focus' face in shadow, which can be useful in scenes which require a character to hold some kind of hidden evil intent or to show they have a 'Dark side'.
The Fill light should be placed so that the other side of the persons face, however it shouldn't be as powerful as the Key light, as it is a light used to fill in the shadows. This can be done by moving the light further back or by using a lower voltage in the bulb. This is useful when attempting to make a character stand out, as their face is completely in view. It is sometimes also useful in horror movies, as it can be used to show a villains mangled or ugly face, alongside the darkness that lurks in the background of the shot.
The Back light should be placed above the focused person although not directly behind and it must not give the camera glare. this can be achieved by placing the light higher. It is used to give the person in focus an outline of light, which can be used to show a character falling in love with the character in shot, as it shows them in an almost angelic form.
