So let's start with the overall basics, and then dig a bit deeper. TSW does not have any classes, but it has 525 unique abilities. When we say that these abilities are unique, we really mean it. We don't have rank 1,2,3,4 etc of an ability. The ability is unique, and it will scale with your gear. A player can potentially learn every single ability in the game. There are no crossroads where the player has to choose either left or right. There are no choices a player can make that would remove his options to purchase an ability at a later date.
Bruusgaard also discusses the ways in which players can combine skills for better synergy, abilities without weapon requirements and the role of crafting. Read the full blog for all the details.
Source: The Secret World Dev Blog