Gucken vs sehen

What is the difference between "Gucken" and Sehen" and in which situation is it correct to used each of them?

Hallo RexV, 

"Gucken" would be translated as "to peek" or sometimes "to see" in a more playful manner, whereas "sehen" only means "to see". 

Here are three examples:

Ich gucke durch das Schlüsselloch. - I peek through the key hole.
Gucken wir mal, ob das klappt. - Let's see if that works. 
Ich sehe besser, weil ich eine neue Brille habe. - I can see better, because I've got a new pair of glasses.

I hope that clears it up for you. 

Beste Grüße, 
Leah