The Wiki tool is used to create and share a collection of student-generated pages within a course site. A wiki can be used for any number of purposes, but is most commonly used for the production of class assignments. Instead of turning in a Word document or PowerPoint file, a group of students can build a wiki (set of web pages) that includes rich media, documents, and web links. The wiki can then be opened to others in the course for comment and discussion, if desired.
It's important to note that while the Wiki tool encourages collaboration among students, it's not the same as building a shared Word or PowerPoint document in Office 365. The wiki tool excels at small group work, not collaboration by hundreds of students in a single wiki page in real time. If you need to have students sign up for groups or topics in your course, please use the Signup Sheets tool instead of the wiki.
Wikis are not true, real-time collaborative environments. That is, you cannot see someone else editing a wiki page and watch them make changes in real time. All wikis, including Wikipedia, operate on a "last person who saves wins" principle. That is, if student A starts editing a wiki page, and student B then begins to edit the same page, and student A saves her version of the wiki page, and then student B saves his version of the wiki page, the version from student B is the most current one, and the one that everyone will see. Student A's changes will not appear in the current version of that wiki page because her changes were saved before student B's. Student A's work is not lost, however, as her work appears in a version of the page in the wiki history. Her work simply does not appear in the current version of the wiki page because student B saved his work and overwrote hers.