The below post was taken from the Elder Scrolls Online Guide from KillerGuides with approval. Several more detailed chapters for the nightblade are waiting for you to discover. It's by far the most detailed guide I have read when it comes to how to level up your nightblade.
Elder Scrolls Online has revealed several classes which players can choose at the beginning. Classes define a warrior or mages’ abilities in battle as players progress through their skill tree. In ESO, skill trees are modeled differently from traditional mmos. The game does contain classes but skill trees are open. Player can choose to unlock abilities used by another class to mold into a powerful skill rotation.
Here are the list of known classes to appear in The Elder Scrolls Online: