Skill points

Skill points is a common term to define the effective level of a skill in Diablo series, although only Diablo II has skill points as such.

Diablo/Hellfire
Each of the classes in the first Diablo possesses a skill that is, for the most part, unique to that class. As they grow in power, so too does their class skill. In this manner: The Monk and Bard classes in the Hellfire expansion are, for the most part, exceptions to this ruling, as the Bard's skill is arguably the best class skill present in the game, while the Monk's class skill is available as a book.
 * The Warrior's Item Repair skill inflicts a lower maximum durability loss the higher his level is, up to a minimum of -1.
 * The Rogue's Trap Disarm gains a better chance to disarm traps the higher her level is, although her Dexterity can also help in this.
 * The Sorcerer's Staff Recharge skill inflicts a lower penalty to a staff's maximum amount of charges the higher his level is, up to a minimum of -1.
 * The benefits of the Barbarian's Rage skill are multiplied by his own level, as are the penalties that are accrued when the rage ends. The duration during which the respective effects for both phases remains at a stationary 12 seconds, however.

A character's knowledge of the spells that he/she knows is measured in spell levels, which in turn, are defined by the amount of books the character has read. To increase one spell's level by one, they must find a book for that spell, and possess the Magic requirement needed, which generally goes up by a multiplicative 20% per level of the spell (i.e, to advance from level 1 to level 2, the character must have 20 Magic, while the next level requires 24 Magic). An Enchanted Shrine also affects a character's knowledge in all of the spells that is already known. A character's innate knowledge of a spell may never exceed level 15 at any point. However, some special items, most notably the Bovine Plate, may further modify spell levels when equipped, either in a positive or negative fashion. Positive spell levels from equipment can go over the cap of level 15. Note that there can be times when a spell may be brought to level 0, which makes it unusable under any circumstances.

Diablo II
In Diablo II, all characters have a preset skill layout in form of a Skill Tree, and skills must be learned by investing skill points in them. There are multiple restrictions to this system:
 * One cannot add more than 20 skill points to one skill. While the level of the skill itself is not limited (i.e. passive bonuses from items, Skill Shrines etc. have no cap), only up to 20 skill points may be invested in a single skill.
 * Synergies grow in power as more skill points are invested in other skills linked to the chosen skill.
 * Some skills have skill requirements: a character must have at least one skill point in all prerequisite skills before they can put a point in the proceeding skill.
 * All skills have a character level requirement, which must be met before the first skill point can be put in a skill. Moreover, if a skill requires level X, to invest another +1 skill point in it, player must achieve level X+1, and level X+Y to invest Y points. To invest all 20 points into a level 30 skill, one must achieve a minimum level of 50. Even if a player has spare skill points, they still cannot all be put into a single skill until certain levels.
 * In later versions of the game, one may reset all skill points at Akara's place, redistributing them as they see fit.

Skill points can be earned by:
 * Leveling up. You will get one skill point;
 * Completing certain quests (Radament's Lair, Fallen Angel and Den of Evil). 4 skill points will be gained per difficulty.

It is advised to think in advance what kind of character one wants to play, keeping in mind that skill allocation determines a build, and vice versa.

Also, ideal equipment (specific set items, runeword armor, etc.) must be selected in accordance with chosen skills.

At Level 99 a character will have 110 skill points, if they have completed all skill quests on all 3 difficulties.

Diablo III
Skill points do not exist in the third game, instead all skills are interchangeable for free, and have Skill Runes to choose from. However, players may only equip 6 active and 4-5 passive skills at once, effectively bestowing limits similar to those of the old system.