A weblog (usually shortened to blog, but occasionally spelled web log) is a web application presented as a webpage consisting of periodic posts, normally in reverse chronological order. The term blog came into common use as a way of avoiding confusion with the term server log.

Blogs run from individual diaries to arms of political campaigns, media programs and corporations, and from the writing of one occasional author to the collaboration of a large community of writers. Many weblogs enable visitors to leave public comments, which can lead to a community of readers centered around the blog; others are non-interactive. The totality of weblogs or blog-related websites is often called the blogosphere. When a large amount of activity, information and opinion erupts around a particular subject or controversy in the blogosphere, it is sometimes called a blogstorm or blog swarm.

The format of weblogs varies, from simple bullet lists of hyperlinks, to article summaries with user-provided comments and ratings. Individual weblog entries are almost always date and time-stamped, with the newest post at the top of the page, and reader comments often appearing below it. Because links are so important to weblogs, most blogs have a way of archiving older entries and generating a static address for individual entries; this static link is referred to as a permalink. The latest headlines, with hyperlinks and summaries, are offered in weblogs in the RSS or Atom XML format, to be read with a feed reader.

A weblog is edited, organized and published often through a content management system or CMS.