A blog is simply a "web log", an online journal created by an individual or company. Each entry, called a post, can contain text, images, video, web links, or any combination of these. Bloggers (people who blog) use special programs to create their blog entries and to add them to the blog.
The blogging program typically adds the date and time to the post before adding it to the blog, and keeps the most-recent posts at the top of the blog. The subject of a post can be anything the author(s) wants, though blogs typically follow a theme or topic: companies may blog about public relation matters such as new products, new hires, and new offices; individuals may blog about cooking, politics, their dog, their family, their careers, or any other topic that interests them.