A wiki is a collaborative web based information databases - simply put, it's an ever growing way for users and site members to create, organize and edit content themselves. Wikipedia is the obvious example - with other web based wikis popping up.
It may seem ridiculously easy to research online, when a Wikipedia entry pops up for whatever you type into a search engine - but hold fire before adding this into your work. A wiki’s information is normally not a good source for academic research as the content may not have necessarily have been edited by a professional. The writer(s) of the wiki themselves may have copied and pasted chunks of other people's work without crediting them (plagiarism), or may be writing from their own perspective - so views could be biased. And modern university software will require you to run all assignments and essays through plagiarism detecting programs before submission - so if you've borrowed heavily from Wikipedia or online sources you can't cite, this will be flagged up.
The Learning Centre has more information about copyright issues here and you can find out how to cite correctly on the Learning Resource Centre Moodle page.