A wiki (from the Hawaiian word 'quick') is a webpage that anyone can edit at anytime from anywhere. Wikis allow for asynchronous collaboration and communication between groups of people. On a wiki many people can edit the page and many people can read the page.
A wiki can imitate the way that you surf the web -- you can add pages in all directions and expand as needed through internal wiki page links and external links to outside sources.
Instructional wikis
Post information for your students to reference outside of the classroom. Wikis are accessible from anywhere, so your students can easily log on to find the most up to date class information. Post your syllabus, homework, vocab list, additional homework resources, inspirational videos, office hours...
On an instructional wiki you are the only editor, your students can read the information but can't contribute.
Collaborative wikis
Create interactive lessons where students contribute to the content on your wiki. Students create, publish and post work. On a collaborative workspace students can peer edit and comment on other student's work, work collaboratively and have easy access to a variety of links.
On a collaborative workspace, students can edit the page and contribute to the content on the workspace.