Pokémon Ruby and Sapphire/Evolution

Evolution is when Pokémon change forms. There are many different ways this happens:

Leveling-up
Sometimes when a Pokémon levels up it evolves. To grow levels your Pokémon must get a certain amount of experience points (exp). Obviously the only way to do that is to have it participate in battle (EXP share also works).

Trading
Using a GBA link cable you can trade certain Pokémon to other trainers. Some Pokémon evolve when this happens.

Items
When some Pokémon are given items they evolve.

Trading with Items
Rare items are known to evolve Pokémon but only if they are traded with the item

Stones
Leaf, Sun, Moon, Fire, Thunder and Water Stones also evolve Pokémon, but these are rare on Pokémon Ruby and Pokémon Sapphire.

Random
Random evolution occurs when one Pokémon may turn into one of two different Pokémon.

Specific-Conditions
When it doesn't fit one of the other catergories it goes here

Taming
When Pokémon are happy enough they might evolve. In Verdanturf Town there is a lady who can rate the happiness.

Spectral
Once one Pokémon becomes two it enters this category.

Combination
If it fits 2 or more catergories it goes here.