Skill Trees

A Skill Tree is a kind of a categorized spellbook where players can see what skills are available to their character.

Overview
Skills in Diablo II and Diablo III, unlike the first game, are tied to certain categories. A skill can only be used when learned (equipped or invested skill points into), and the total number of skills character may have at a time is limited (unlike the first game, where one could, in theory, learn all spells that exist in the game world).

There are many ways to categorize skills, such as by Elemental Damage type, by being tied to certain game mechanics, etc. They can be divided by the degree of action required to use them:
 * Active: most numerous skill category; to get an effect, the skill must be activated manually, usually one cast per trigger.
 * Channeled: must not only be activated, but also maintained for the entire duration, most other actions (especially movement) being unavailable while doing so. Usually the skill is maintained by holding the cast button continuously.
 * Summon: creates a minion, pet, construct or any other independent entity which will fight on player's behalf. Most summoned creatures disappear if their master dies, and may be killed by enemy attacks.
 * Passive: do not need to be activated; once selected or learned, will always provide their benefit. The bonus may either be permanent, or trigger at certain point (such as Life falling beneath a certain threshold).
 * Active / Passive: these skills combine traits of both active and passive.
 * Auras, Laws, Mantras and other similar traits often have a passive effect, but can be activated manually to receive an extra temporary bonus, usually once in a while.
 * Buffs are applied manually, but once cast, provide only passive benefits to the player, and do not need to be cast in combat (except for refreshing their duration, which is usually long enough to survive all but longest battles without recasting it). Most Buffs are lost upon death.

Diablo II
In Diablo II, it has three tabs at the right side. Clicking on the tab will take player to the corresponding skill tree.

The tree consists of rows of skills. The row indicates the level requirement to acquire the skill. The first (top) row of skills are available as soon as the character is created. The second row has a level requirement of 6, with each following row's level requirement increasing by 6, up to 30 for the bottom row.

Some skills have one or more arrows pointing to them. Any skill on the tree that points to another skill is a prerequisite. For example, in the image to the right, Fire Claws has arrows pointing to it from Feral Rage  and Maul. This means that in order to get Fire Claws, one must already have at least one skill point in both Feral Rage and Maul.

Skills synergize with each other, passively enhancing other skills, usually in the same tree.

Development
The idea for skill trees in Diablo II came from David Brevik; according to Brevik himself, the idea came to him while he was in the shower. He likened the idea to the tech trees of Civilization II.

Development
In the early versions of Diablo III, skill trees were working in the same way they did in Diablo II: three separate trees, unlocking more passive and active skills as players invest more skill points in them (also similar to World of Warcraft pre-MoP design). However, eventually this concept has been canceled, although most of the skills were more or less transferred to the new system of skill categories (see below). This allowed making mixed builds and prevented players from wasting points on skills they do not want just to get to the next skill in the category. However, details about two older skill trees (Barbarian and Wizard) still exist:

Current System
In Diablo III, skill trees were replaced with skill categories. Each class has six, and each category includes three to five skills. The system below is approximate, in fact, Demon Hunters and Witch Doctors have their categories slightly different from those of other classes. s have their categories totally different, and grouped mostly by the type of resource these skills use.
 * Primary (default LMB): basic skills that do not waste resources, or even generate them with every attack, have no cooldown, but deal less damage than other skills. Pants, Class-specific Items and Belts may enhance the damage done by these skills.
 * Secondary (default RMB): advanced skills that usually waste resources, but generally have no cooldown as well; basically, they are the main combat skills. Helms, Boots and Class-specific items may enhance the damage done by these skills.
 * Defensive (default 1): protective skills that are focused on protection or on disabling enemies (i.e. Crowd Control) rather than dealing damage. They usually cost no resources (or even generate them), but have cooldowns, and while some of them deal minor damage, their main purpose is survival.
 * Tertiary (default 2, name of the category varies by class): powerful skills, usually they cost more resources than Secondary, and / or have cooldowns. These skills mostly deal damage, are considerably more potent than Secondary skills, but players won't be able to use them as often as they can use Secondary skills. Boots, Class-specific items, Pauldrons and Chest Armors may enhance these skills.
 * Special (default 3, name of the category varies by class): these skills are usually semi-passive, enhancing the character and / or allies, or summoning pets and minions. Some classes can only pick one skill of this category at a time. However, these skills rarely do damage. Class-specific items, Pauldrons and Chest Armors may enhance these skills.
 * Ultimate (default 4, name of the category varies by class): these skills are usually the most powerful tools in the character's arsenal. They can be incredibly damaging, very potent Crowd Control forms, enhancing the character far beyond their usual capabilities for a short time (or even changing their physical form into a creature of legends) or summoning very numerous or very powerful minions. However, most of these skills have a very high resource cost and / or a very long cooldown. Class-specific items, Pauldrons and Chest Armors may enhance these skills.

Passive skills have a separate category. At level 10, a character has one passive skill slot, but can acquire the second one at level 20, the third one at level 30, and the final (fourth) one at level 70. Hellfire Amulet allows having five different passives at a time. Passive skills can be taken in any combination, regardless of which active skills have been chosen.

Originally, characters can only have one skill of each category, and may not change the default button. However, with the so-called Elective Mode turned on in the Options, one can take any skills in any combinations, unless specifically stated otherwise (such as Wizards only being able to use one Armor spell).

Skills can be changed or assigned new Skill Runes (counts as changing skills too) at any moment when out of combat, and when the skill in question is not on a cooldown. If performed in town, the new skill is ready instantly, otherwise it has a 10 seconds cooldown before it can be used.

Note that each item can only have one "Increases (Skill Name) Damage by X%" Primary affix.

Diablo IV
Skill Trees return in Diablo IV.

Following the initial criticism of the Talents system that essentially forced the player to choosing one of the pre-determined paths (with just a few alternate selections, mostly in the end), the entire system was revamped to allow more liberty, fashioned in the form of an actual tree.

Skill Trees are divided between 'branches' and 'roots.'
 * Branches contain skills and skills upgrades, and use the Skill points. Skill points put into skills unlock new active abilities, increase their potency, and there are also upgrade nodes that provide additional effects. Each node leads to distinct options, literally branching the choices, but unlike Talents, nodes are not mutually exclusive. Branches are not necessarily tied to a single skill type: i.e. skills in a branch do not lock the character to a specific play pattern (for example, may have different types of Elemental Damage in a single branch).
 * Roots contain powerful passive effects, using Passive points. Roots are similar to the old Talents (and reincorporate most of them), but are not locked into strict chains. Root nodes are general, increasing overall powers not tied to a particular skill.

Players are not able to pursue every branch on the tree. The developers intend that 30-40% of nodes can be filled in by the end game.