The word Blog got its name from its original moniker - "weblog." Blogs started out mostly as the online journals of hobbyists and aficionados. It was a way for people to socialize online and share what might have previously only been expressed in a locked diary. The rapid growth of blogs was first felt in the political arena.
There is no technical definition of a blog. Any website can function as a blog, though most blogs have similar traits and features. Usually blogs are displayed as a sequential posts sorted by date, the most recent on top.
There are many popular blogging tools, most of them free, including WordPress, Movable Type, TypePad, Blogger (which is currently struggling with significant spam issues), and Xanga.
Blogs usually allow readers to add comments to a post. These days, to avoid comment spamming, most blogs require either registration, or a "captcha" field (which requires poster to enter the graphic letters into a field - something an automated spam bot can't do). In order to minimize the number of user names and passwords we use for commenting on blogs, some blogs have adopted the use of TypeKey. Setting up and logging into a TypeKey account grants you commenting access to all blogs that support TypeKey accounts.
Some blogs post all comments so long as registration or captcha entry is successful. Others require editorial review and approval before posting.
Blogs grow in popularity in part by how many others read, link to, and subscribe to them. Therefore they often include "link bait" for each post. These are short cuts that allow readers to add the posts to their bookmarking services (like del.icio.us, Diigo, and Furl) or recommend it to news and blog aggregators (like Digg and Technorati - more on Technorati later).
Most blogs allow readers to subscribe to the blog by receiving new posts in their RSS reader (you have an RSS reader, right?).
Blogs typically include a right column containing navigational and indexing tools such as latest posts, archives, search, tags, and blog roll (other blogs that blogger reads and recommends). Sometimes this column includes a "recently read by" widget using the MyBlogLog service.
Aside from the visual elements that make up a typical blog, there is something more fundamental that defines blogishness. That is, its culture of openness and transparency. Even though many blogs and blog comments are not personally identifiable (they're usin' code names) the nature of the postings is often very personal and transparent. Bloggers are not afraid to let the world know what they think, what they like, what they read, and what they listen to. In fact, the latest trend in blog accounts is to use real names rather than aliases. I've opted for this approach, so if you ever see my comments on a blog you'll see my real name next to it. I find that being transparent about identity increases the weight a comment since it's not speaking from behind a veil of anonymity. It also makes me a little more conscientious about my language and tone before posting since I'm using my real name. I think this bit of accountability improves the quality of posts and comments, which benefits everyone.