A wiki is a type website or software that allows people to easily add, remove, or edit some available content. This all happens within the website or software without the need of users actually having copies of the content on their hard disks. This, along with the ease of interaction, allows wikis to be a good tool for collaboration amongst a group of people.
Wikis usually have a simple and easy to use markup language that is used to create and edit "wiki pages". These pages can also link to one another with simply formatted hyperlinks. (usually ref links).
The idea of a wiki is simple: make a web site editable directly from your browser. No need to install any software, no need to apply for permissions, no need to email a document for someone "to be put on the intranet". You just click on a link, and edit the page.
The concept takes a bit of getting used to, but it does really work! The benefits of having an easily updated web site far outweigh the potential problems of people writing "improper" content. If they can write bad content on your wiki, it's also easy for you to remove it.
And of course, if Wiki is used in your corporate intranet, typically you can track the usage of the wiki..