I think these definitions are great for Blogs and Wikis.
Blogs are mostly (but not exclusively) about the author. Blogs are a form of communication - the "voice" of the people posting entries. Wikis are about a shared space and are mostly (but not exclusively) about the content (the people are more on the periphery, content has the center stage). Overlaps are inevitable and there are gray areas (e.g., blogs that are embedded within applications).
Blogs are more about communication and are author-centric. Wikis are more about collaboration and are content-centric. The open communication of both blogs and wikis is much more than a technology and Enterprise 2.0 is much more than a technology play.
Source: Portals and KM: Defining Blogs and Wikis: Technology or Style?
There are clearly technical differences but essentially a "blog view" or "wiki view" is enabled by the underlying hypertext platform. A more technical perspective is provided by Jordan Frank below:
And that, finally, brings me to a baseline definition for both blogs and wikis:
A system for posting, editing, and managing a collection of hypertext pages (generally pertaining to a certain topic or purpose)...
Blog: ...displayed as a set of pages in time order...
Wiki: ...displayed by page as a set of linked pages...
...and optionally including comments, tags or categories or labels, permalinks, and RSS (or other notification mechanisms) among other features.
Both "blog" and "wiki" style presentations can make pages editable by a single individual or editable by a group (where group can include the general public, people who register, or a selected group). In the enterprise context, more advanced version control, audit trail, display flexibility, search, permission controls, and IT integration hooks may also be present.
Source: Traction Software, Inc. | Blog372: What is a Blog? A Wiki?
I found this summary on Portals and KM: Defining Blogs and Wikis: Technology or Style? on Mike Gotta's Collaborative Thinking blog. Good stuff Mike.