Evolution is a mysterious phenomenon. When a pokemon meets a certain requirement, it will evolve into an entirely different species. Evolved forms are more powerful and have different bodies. The pokemon may even change types, or spontaneously learn a new move.
The most interesting pokemon when it comes to evolution is Eevee. It has the capability to evolve into many different pokemon due to its unstable genetic makeup. One such evolution is Leafeon. If an Eevee holds a Leaf Stone, it will use the stone up and evolve into Leafeon.
The methods for evolution vary among different species of pokemon. One method, the most common method, is for the pokemon to reach a certain degree of strength. Other methods include the aforementioned holding of a special item and the learning of a certain move.