Common Evolutions
There are many different types of evolution in the Pokemon series. The most common type of evolution is the Level Up evolution. Once a Pokemon finishes a battle that takes it to its requisite evolution level, the Pokemon will evolve. Other types of evolutions are Trade evolutions and Item evolutions. Traded Pokemon sometimes evolve during the transfer process from one player to another. Item evolution requires a specific item be used or held during a trade for the evolution to trigger.
Nincada's Evolution
At Level 20, Nincada experiences a normal leveling evolution. Nincada evolves into Ninjask, going from Bug/Ground-type to Bug/Flying. What makes Nincada's evolution special is what happens when you have an empty slot in your party and a spare Pokeball. If you meet these two requirements, after the evolution you will find an extra Pokemon in your party. The carapace shed by Nincada comes to life and becomes a Shedinja.
Shedinja
Shedinja is different from other Pokemon in several ways. Shedinja is the only Pokemon with the Bug/Ghost type. Also, because it simply appears in your party, it is the only untraded Pokemon that you are not given a chance to name upon capture. However, Shedinja's HP stat is the first thing you may notice about it. Shedinja has only a single health point, which cannot be raised.
Wonder Guard
Shedinja makes up for its HP weakness with its Special Ability, Wonder Guard. Wonder Guard makes it completely immune to all non-super-effective moves. Shedinja can be hurt only by moves of the Fire, Ghost, Flying, Rock and Darkness types. However, Wonder Guard does not protect from indirect damage sources, such as status effects, damaging Special Abilities and held items. Because Wonder Guard does not protect from other Special Abilities, the Mold Breaker ability can negate the effect of Wonder Guard. Mold Breaker can only be found on Pinsir, Cranidos and Rampardos.