Shape Keys are usually used for animations, but they can be quite useful tool while you are modeling. Let's say you're modeling something like this little toy robot and you're maybe not sure if it looks right, with the Shape Keys you can experiment with your model without changing the original shape.
Let's use the Shape Keys to play around with his ears. First select the object you want to change and then press the small plus button in the Shape Keys tab. we created the first Shape Key called Basis, we won't do anything with this Shape Key, its only purpose is to save the original shape of our model.
If you go back to object mode you will see that your model switches back to its original Shape, don't worry you did nothing wrong. Every Shape Key you make has a slider that goes from 0 to 1, by default the slider is at 0 and that's why you can't see any change. You can make as many Shape Keys as you want and of course you can combine them, This is a very easy way to experiment with the Shape of your model until it looks perfect.
Now we have a new Shape Keys which is a combination of all the others, if you want to keep the new Shape but you want to get rid of all the Shape Keys then just delete the Basis and all the other Shape Keys besides the one that is active. You can delete this one too, just be sure it is the last one left before you delete it.
I used this method to make a huge collection of Vases. In a very Quick and easy process I managed to create over 70 different looking models. If you're interested you can click here to take a look at the collection.