What is a Wiki?
a technology for collaborative creation of internet and intranet pages
Wiki-Wiki is hawaii and means fast
Principles
Fast - fast editing, communicating and easy to learn
Open - everybody may read and edit everything
Uncomplicated - everything works in a standard browser
Easy - no HTML knowledge necessary, simply formatted text
Simple - Content over Form (content counts, not the super-pretty appearance)
Secure - every change is archived, nothing gets lost
Interlinked - Links between pages are easy to make
Flexible
Personal Information Management, Knowledgebases, Brainstorming
Collaboration, Coordination and Communication platform
Creating documentation and slide shows
etc...
For details see HelpForBeginners.