"A wiki ( /ˈwɪki/ WIK-ee) is a website that allows the easy creation and editing of any number of interlinked web pages via a web browser using a simplified markup language or a WYSIWYG text editor. Wikis are typically powered by wiki software and are often used to create collaborative wiki websites, to power community websites, for personal note taking, in corporate intranets, and in knowledge management systems."
Wikipedia is useful but not 100% accurate, I often use it to find information or an explanation of something as it is quick and easy to navigate.
As for using wikis in library work, at the Education library we have our own wiki, which we use to compile information and lists. Particularly useful is a 'things to do' section that has a list of tasks that need to be completed. Colleagues can cross things off once they're done and it's all in one place so there's no confusion.