As of version 1.52, milestones were removed from the game. While they were active they allowed players to pick one of two traits every ten levels such as increased physical damage or access to a certain ability. A skill tree system which allows for a much greater degree of class customization has since taken their place. More detailed information about skill trees can be found by heading to the class list page, then choosing one of the available classes.
As of version 1.63, milestones were reintroduced as part of the customization for companions. Much like their original form, milestones allow players to choose between one of two traits for their companion with the choice coming every five levels instead of every ten levels. A list of milestones can be found either on a monster's information page alongside its wild traits or via the Companions page.