He likened the idea to the tech trees of Civilization II. 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.

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).

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.

Active / Passive: these skills combine traits of both active and passive.

The bonus may either be permanent, or trigger at certain point (such as Life falling beneath a certain threshold).

Passive: do not need to be activated once selected or learned, will always provide their benefit.

Most summoned creatures disappear if their master dies, and may be killed by enemy attacks.

Summon: creates a minion, pet, construct or any other independent entity which will fight on player's behalf.

Usually the skill is maintained by holding the cast button continuously.

Channeled: must not only be activated, but also maintained for the entire duration, most other actions (especially movement) being unavailable while doing so.

Active: most numerous skill category to get an effect, the skill must be activated manually, usually one cast per trigger.

They can be divided by the degree of action required to use them:

There are many ways to categorize skills, such as by Elemental Damage type, by being tied to certain game mechanics, etc. 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).

Skills in Diablo II and Diablo III, unlike the first game, are tied to certain categories.

Diablo 2 skill calculator deutsch