According to the Wikipedia, a "wiki" is an Hawaiian word for "quick." Thus, a wikipedia is a "quick encyclopedia. " The main feature of a "wiki" is that the software allows for multiple users to creative a collaborative workspace. The Wikipedia is a collaborative encylopedia written by many users.
It allows people to build a shared knowledge space. Particularly for those educators who have been interested in contructivist-based approaches for teaching this ability to create a shared knowledge base allows educators to involve students in the construction of knowledge or information about an area of study and then provide feedback and elaboration of this information. (See How People Learn for a complete explanation of the current research on designing learning environments.)