Nail Extensions vs. Overlays: What's the Difference?

Question
What is the difference between nail extensions and nail overlays? I have seen both on various salon price lists but don't know the difference!

Answer
A nail extension is when you extend the lenght of your nails, either by applying a plastic tip and then the enhancement product (gel, acrylic, fiberglass), or by sculpting with the product.
An overlay is when you apply a nail enhancement product on your natural nails to make them stonger wich alows them to grow longer.