Skill tree is a UI element which allows to assign skill point to skills.
Some games allow skills to be "equipped" with objects or items.
Typically there are several areas of skill trees:

Offensive skills: Skills which require casting/attack and deal damage

Passive Skills:provide 24/7 stat/effect boost for the character.

Buffs/Auras/State change skills:provide defensive/offensive buffs/state changes that can be selected(Aura) or cast(Buff).

Minions/Summon: Minion/Pet/Summon skills