Evolving is a process in which a Pokemon permanently changes into a more advanced form. Not only will the looks of the Pokemon change, but various other attributes will/can as well, such as type, ability, and stats.
Evolution will occur when certain requirements are met. For example, the Pokemon Charmander will evolve when it reaches level 16.
Evolving your Pokemon is permanent, so decide wisely.
There are more ways to evolving Pokemon, such as:
Leveling up is the most common method to evolve most Pokemon. Simply training a Pokemon until it reaches a certain level may evolve it. A specific known move or held item is required sometimes.
Trading a Pokemon can trigger an evolution. In some cases, the Pokemon being traded must have a certain held item, depending on what kind of Pokemon it is, and in even rarer situations, the Pokemon must be traded in return for another certain Pokemon.