Said customization is "almost unlimited. NPCs in the game are actually made by not modeling but customizing each. Blade & Soul will provide the most free customizing system ever so far," according to the devs. It's somewhat unclear as to whether this customization will take the form of character skills and abilities or physical differentiation via body types and personalization options.
It's also worth noting that some of the article's information may have been lost in translation, and if your Korean language skills are up to the task, you can read the full original text on ThisIsGame's main site.