Evolution is a way for player characters (pets can not evolve) to advance themselves by become a new class or race.
Class evolution
You can evolve your class at level 50, 350 and 650 it is recommend you evolve as soon as you get to these levels as if you leave it later than this you will miss out on stat gains.
When you evolve into a new class you will gain new skills and improved stat gains per level, you may also gain access to better types of armor.
At level 50 you will have three class choices (one if you are playing a bought class). Pick your class wisely as once you choose you are locked into the future evolutions of that class and can not jump between evolution lines.
To view the class options and skills you may gain when evolving go to Library > Profession > Profession Evolutuion.
Race evolution
You can evolve your race at level 200, 400, 600 and 800 as with class evolution once you make your first race evolution choice you are locked into the future evolution for that race so you should choose wisely.
Race evolution is primarily intended to get a cool new picture so you are different from other players but it also gives a one time bonus to specific stats so it can be a cool way to improve your character.
To view the races options and stats you may gain when evolving go to Library > Races > Profession Evolutuion.
General notes on evolving
To evolve into a new class and/or race you must meet that evolutions requirements, these may range from specific amounts of skill to a specific amount in one stat or any combination of the two.
If you do not meet the requirements you can not evolve.
Where do I evolve?
There are buildings dotted around the world that have Evolved Spirit NPCs inside who can help you reach enlightenment (evolve). There is one such building at 32, 62 in Knotwood Valley called South Glade Tower (if you do not see it then it needs to be explored).