In ESO it works like that (only an example):
select a class Sorcerer --> get access to the skill tree (skill set or what ever) "Sorcerer"
select a race Altmer --> get access to the skill tree "Altmer"
while playing:
equip light armor (at least one piece) --> get access to the skill tree "Light Armor"
equip destruction staff --> get access to the skill tree "Destruction Magic"
fulfill some requirements and become a vampire -->get access to the skill tree "Vampire"
join Mages Guild --> get access to the skill tree "Mages Guild"
and so on
From all those skill trees you can choose up to 6 skills which you want to use at a time.
If you switch to a sword you can no longer use "Destruction Magic" until you equip a destruction staff once again. But intead you get access to the "Sword" skill tree.
Some skill trees exclude each other. You can't be vampire and werewolf. (At least not at the same time, maybe there is some way to change that. I don't know)
Maybe Mages Guild and Fighters Guild are like that, too.