It is maximum level of skill acquirable without items. It varies between 6 and 16.
It is maximum acquirable skill level. Due to item bonus Hero can obtain level 4 higher than the skill Maximum Level.
Active Skill - Hero must intentionally use such skill to invoke its effects. These effects are instant, but may have duration components.
Passive Skill - Such skill provides benefits all the time. If one meet the specified criteria ( using right type of damage, etc. ) the skill works automatically. It may have a chance to proc or be a permanent bonus.
Modifier - It is specific type of Passive Skill which works only with conjunction with its base skill.
Sustained Skill - Once activated, this skill remain in effect until Hero turns it off or run out of Energy. Such skills may lock some of Hero's maximum Energy or have Energy per Second cost. They usually have some sort of radius component.
Some skills cannot be used as frequently as desired - they have cooldown - time period after recent usage when skill is unusable. One is free to use other skills, while one of them is on cooldown.
Some skills are connected together and form a larger groups (which - in most cases - contains one Active Skill and Modifiers to it). Purchasing further skills from such group requires having at least the first, lowest one and sometimes even all preceding skills.