A skill progression tree or simply “Skill Tree” is a system common in video game RPGs and some table top RPGs consisting of a branching hierarchy organization for skills or abilities or whatever they are called in the game.
Typically the game will present players with an entry level skill or series of skill. That skill then branches out with items like supplemental skills as well as active or passive effects that are made available as the player progresses through the game.
Here's an example:
Sup Skill Passive
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Active Passive Sup Skill
Progression through the tree is usually handled in one of two ways. When the player reaches a branch split they must choose which direction to follow with the other branch getting locked out. Conversely, the system may be set up so that the player is awarded points which may be used to purchase items on the tree as long as they possess the prerequisite root item(s). This second method allows the player to gain an item from anywhere on the tree they can “legally” purchase, but the game has a maximum amount of points available so that the player cannot purchase the entire tree.
The purpose of a system like this is to allow players to customize their character to suit their play style. It also induces a somewhat organic difficulty curve to the game, since the player will be good at some things and bad at others. The game creators can put in a standard difficulty for all those things; that way multiple play throughs using different skill paths will change the feel of the game.
Variations of this system in use can easily be seen in games like Diablo, World of Warcraft, Dragon Age II, and D&D 4th.
--An August FactQuest--