The definition of Blog

The term web-log, or blog, was coined by Jorn Barger in 1997 and refers to a simple webpage consisting of brief paragraphs of opinion, information, personal diary entries, or links, called posts, arranged chronologically with the most recent first, in the style of an online journal . Most blogs also allow visitors to add a comment below a blog entry.
This posting and commenting process contributes to the nature of blogging (as an exchange of views) in what Yale University law professor, Yochai Benkler, calls a ‘weighted conversation’ between a primary author and a group of secondary comment contributors, who communicate to an unlimited number of readers. It also contributes to blogging's sense of immediacy, since ‘blogs enable individuals to write to their Web pages in journalism time – that is hourly, daily, weekly – whereas the Web page culture that preceded it tended to be slower moving: less an equivalent of reportage than of the essay.
Each post is usually ‘tagged’ with a keyword or two, allowing the subject of the post to be categorised within the system so that when the post becomes old it can be filed into a standard, theme-based menu system2. Clicking on a post’s description, or tag (which is displayed below the post), will take you to a list of other posts by the same author on the blogging software’s system that use the same tag.
Source; Paum Anderson ,What is Web 2.0?: Ideas, technologies and implications for education, p7