Condensed from Wikipedia -
A blog (short for web log) is a discussion or informational site published on the World Wide Web that consists of individual "posts" typically displayed in reverse chronological order (the most recent post appears first). Blogs can be single or multi-author. Twitter and other "microblogging" systems help push blogs into newstreams and popular discussion. Blog can also be used as a verb, meaning to maintain or add content to a blog.
The emergence and growth of blogs in the late 1990s coincided with the advent of web publishing tools that facilitated the posting of content by those who didn't know HTML. Blogs are interactive with visitors leaving comments. This interactivity distinguishes blogs from static websites, making blogging a form of social networking service.
Many blogs provide commentary on a particular subject; others function as more personal online diaries; others function more as online brand advertising of a particular individual or company. A typical blog combines text, images, and links to other blogs, Web pages, and other media related to its topic. The ability of readers to leave comments in an interactive format is an important contribution to the popularity of many blogs.