"Blog" is just a species of slang for a class of web-journaling applications. It overlaps imperfectly with livejournal, a related but culturally distinct community that tends more towards personal, diary, fannish, and less serious usages. Here's Samizdata's definition.

I personally use Blogger, and I haven't spent a dime on the hobby. You can go from my cheapskate model through various designed and maintained models to the heavy-iron bandwidth-intensive requirements of a Instapundit or a Lileks, with various blogging applications like Movable Type or communities like mu.nu.

As for professional, journalistic uses of blogs, you have a number of variants.

As I mentioned, BBC tends to set up very bare-bones blogs for specific events and occurrences like the Gulf War reporters' log. There's no commenting features, no real use of fancy html tricks, little flash and bang - just a kind of quick, terse, immediate rolling log of reports which could have been produced on teletype or even by telegram.

The next step up from that is the National Review's the Corner, which is technically similar to the BBC's reporters' logs in their lack of commenting and heavier structure, but differs in its persistence, and the breadth of subject matter. NRO uses the Corner to provide a sense of community and identity tying together the "face" of the political magazine with the people who actually produce the magazine. There's a lot of pop-cultural foolishness and linking of "timewasters", mixed in with quick news-bites and a lot of arguing back and forth based on those snippets. In general, the Corner offers the appearance of transparency for its sponsoring organization. It helps deflate the notion of a "Vast Right-Wing Conspiracy" to see John Derbyshire and John Podhertz argue over Star Wars, and Jonah Goldberg's comedic attempts to get around Corner editor Kathleen Lopez's dictatorial ban on Star Trek references. It's the political magazine re-cast as a sort of reality-show sitcom.

The heavy-grade model is that of the Daily Kos and RedState. I'll discuss RedState, because I don't pay much direct attention to dKos, which isn't really my political cup of tea. RedState essentially took their blogging model from dKos's earlier example, anyways.

RedState is run by a board of directors, who are the primary posters on the site - off the top of my head, trevino, Augustine, Thomas, Erick, doverspa, and krempasky, but you could check their director's page to see, and thus I discover, myself, that only Josh Trevino, Mike Krempasky, and Erick Erickson are directors. It's easy to loose track in the audience as to who's exactly in charge, and who's just an editor. RedState started out rather informally, but last year they registered as a 527, and it's definitely an activist's medium. The directors and editors post articles. To comment, a visitor has to register with the site. This registration also allows the visitor to create their own diary entries, and theoretically, the directors or editors can find merit with these and promote them to the main page along with their own output. RedState has a relatively strong editorial policy, and thus has a good reputation for civility and comity. There's no swearing, abuse is frowned on, and "trolls" - people who come in to specifically cause problems and stir up trouble - are quickly banned from commenting and posting diary entries. Some folks use their diary privileges to essentially run their own blogs off of RedState, but I don't generally bother. I've got my own personal space.

RedState also has a Corner-like discussion-log called RedHot. They will spin off additional, themed blog-threads as the politics and events warrant. Currently, the most notable of these is ConfirmThem, a partisan blog dedicated to the ongoing Senate confirmation drama. Isn't really my sort of thing, but as an example...

For blog-usage in journalism in general, Jeff Jarvis of Buzz Machine is probably your man. He's a big blog-evangelist, and is a kind of mainstay on the journalism conference circuit on the subject. He has a lot of opinions on how blogs ought to fit into the professional media. He gets a little technical for my tastes, but then, I'm not a journalist. Jeff's favorite catchphrase is "the news as conversation".