"A Weblog (which is sometimes written as "web log" or "weblog") is a Web site of personal or non-commercial origin that uses a dated log format that is updated on a daily or very frequent basis with new information about a particular subject or range of subjects. The information can be written by the site owner, gleaned from other Web sites or other sources, or contributed by users.
As a format and content approach for a Web site, the Weblog seems popular because the viewer knows that something changes every day, there is a personal (rather than bland commercial) point-of-view, and, on some sites, there is an opportunity to collaborate or respond with the Web site and its participants.
A Web log often has the quality of being a kind of "log of our times" from a particular point-of-view. Generally, Weblogs are devoted to one or several subjects or themes, usually of topical interest, and, in general, can be thought of as developing commentaries, individual or collective on their particular themes. A Weblog may consist of the recorded ideas of an individual (a sort of diary) or be a complex collaboration open to anyone. Most of the latter are moderated discussions."
Currently, the most popular Weblog is Slashdot.org, the product of programmer and graphic artist Rob Malden and several colleagues. Slashdot.org carries discussion thread on many subjects including: Money, Quake (the game), Netscape, Sun Microsystems, Hardware, and Linux. Slashdot.org solicits and posts interesting stories reported by contributors, includes a link to the story, and manages the threads of the ensuing discussion by other users. Another well-known Weblog is Jorn Barger's Robot Wisdom Log, which is more of collection of daily highlights from other Web sites. Jessamyn West's librarian.net is a daily log of items interesting to librarians and possibly others, too.
(Source http://whatis.techtarget.com/definition/0,,sid9_gci213547,00.html)