A wiki uses wiki technology - which creates an online document and repository (or library) where participants can throw out ideas, provide feedback or comment on the work of others, and share documents.

What's a Wiki good for?

Internal Corporate Encyclopedia:  In a corporate environment, a wiki doesn't appear to have much value.  Employees probably wouldn't have much enthusiasm or incentive to contribute to a corporate encyclopedia.

External Collaboration: Some organizations are looking at ways to utilize the vast amount of knowledge out there and interact with others that have knowledge on a particular topic.  A wiki-like technology enables collaboration with outsiders and enables clients, visitors, employees, etc. to collaborate and contribute input.