What Is Blogging?
The term "blog" lacks a single well-accepted definition. Blogs are one of many ways to publish content over the Internet, along with other publication methods such as message boards, chat, email lists, USENET groups, and websites. There are no bright-line distinctions between these publication methods. However, to the extent to which blogs are a discrete Internet publication medium, blogs typically adhere to the following three conventions:
- Reverse chronological presentation. Blog posts almost always are presented in reverse chronological order (with the latest posts on top). 
- Self-edited. Typically, a blogger publishes content without third-party review or editing.
- RSS feeds. Most blogs offer an RSS feed  that notifies readers of new content upon publication.
In addition, blogs often have the following attributes:
- Multiple navigational structures. In addition to chronological navigation, blogs may structure their posts into additional taxonomical structures (by subject matter, author, and so on).
- Personal observations. Bloggers often post their personal perspectives and commentary.
- Interlinking. Blogs routinely link extensively to other blogs in substantive blog posts and via a "blogroll." 
While bloggers follow many norms and conventions,  none of them are immutable—except, perhaps, the reverse chronological presentation of new posts. This fluidity means that any discussion about blogs—including this one—typically applies to other types of Internet publications as well.
Bloggers can work together in a variety of ways. A "guest blogger" typically is given the right to publish content via the blog for a fixed period of time. Guest bloggers typically don’t get administrative power over the blog’s operation. In "joint blogs" or "group blogs," two or more bloggers publish content via the blog on an ongoing basis. Sometimes a subset of bloggers have principal responsibility for the blog’s administration; in other cases, all bloggers share administrative rights.
I regard both guest blogging and joint/group blogging as "co-blogging."