Pokémon Ruby and Sapphire/Evolution

Evolution is when POKeMON change formes. There are many different ways this happens:

Leveling-up
Sometimes when a POKeMON levels up it evolves. To grow levels your POKeMON 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 POKeMON to other trainers. some POKeMON evolve when this happens.

Items
When some POKeMON are given items they evolve.

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

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

Random
Random evolution occures when one POKeMON may turn into one of two different POKeMON.

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

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

Spectral
Once one POKeMON becomes two it enters this catergory.

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