implements is used for interfaces (Yolk implements Cookable)
extends is used for classes (Yolk extends Egg)
Inheritance is an is-a relationship. It is correct to say that a scrambled egg is-a egg. Therefore it would be correct to model the relationship between a scrambled egg and an egg as an inheritance relationship. Scrambled egg would extend egg and would probably expose methods for doing the actual scrambling.
It is not correct to say that a yolk is-a egg. Instead, you would say that an egg has-a yolk. Therefore, it would be correct to model the relationship between a yolk and an egg as a delegation relationship. Similarly, you could say that an egg has-a yolk, shell, and egg white. You could then refer to the relationship as an aggregation or composition depending on which definition of the terms you choose--UML or Gang of Four (GoF).