What is the difference between a blog and a wiki?

A blog is more like a journal or diary. A single author or small group of authors post small news items. Viewers are often welcome to comment on the various posts, but cannot edit the content. A wiki is more of a communal experience. Anyone who is a member of a wiki is allowed to edit the content found within - both in the actual pages and in the comments/discussion field.

What sort of things might be better suited for a blog and better suited for a wiki?
A blog could be used to keep people updated about what is new in the library.
A wiki could be used to facilitate communication with a staff or for inviting patron participation in the creation of a resource.