Each hero has one passive skill and three active skills, the third being the ultimate skill.
Each time a hero levels up in the game, the player gains a skill point, which can be used to upgrade one of the three active skills.
Each active skill has its own cooldown period, which is the length of time that the player needs to wait before the skill can be casted again.
Passive skills are skills that grant a hero certain unique abilities and/or better attributes.
Active skills are skills that can be actively casted, and can deal damage or deliver certain effects to other heroes and/or creeps.