High and Low Key Lighting

High and Low Key Lighting are completely different to one another and which one you use, depends on the kind of effect you would like to aim for. 
For example, the 'high' key effect gives a more calm, airy and bright effect towards the image.

The 'low' key effect comes across more mysterious, dramatic and dark to the image. 

High Key Lighting

High key images comes across more airy and light.
High key photography is where it involves involves large amount of light tones, fewer mid-tones or shadows. 
The best way to obtain high key is by using 'in-camera techniques'.
To do 'high key lighting', using a white, clear backdrop will give the photo a better effect as it's 'high key' meaning bright.

Examples of 'High Key Lighting' -

Low Key Lighting

Low key lighting comes across like dramatic and full of mystery because of the background drop and the dimmed lights on the person or item.
This key lighting gives it a more serious or even moody effect to the picture. 

Examples of 'Low Key Lighting' -