Each base class has a passive tree, and each base class has 3 masteries. You can only master in 1 of the 3. Each mastery has its own passive tree as well. After a certain number of points in a passive tree, you unlock additional skills related to that class or mastery.

Each skill itself (max of 5 skills on the skill bar) had it's own speculation tree. You'll get ~20 points to customize how the skill plays (things like increased area of effect, cast speed, damage element type, etc).

There's a lot of synergy between the skills, so you can specialize them in ways that make them feel very powerful.

Personally, it's my favorite ARPG of the current generation. However it does feels a bit easy. I'm hoping they have it tuned towards getting people to end game faster to get more feedback during early access. I didn't feel like I was where I should be in terms of difficulty unless I was in a zone 10 levels higher than me. Hopefully they narrow that gap by 1.0 and make it a little more difficult.