Blog is a blend of the term web log. It is a type of website or part of a website. Blogs are usually maintained by an individual with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in reverse-chronological order. Most blogs are interactive, allowing visitors to leave comments and even message each other via widgets on the blogs and it is this interactivity that distinguishes them from other static websites. Many blogs provide commentary on a particular subject; others function as more personal online diaries. A typical blog combines text, images, and links to other blogs, Web pages, and other media related to its topic. Microblogging is another type of blogging, featuring very short posts (e.g. Twitter). — Wikipedia
Blogosphere is made up of all blogs and their interconnections. The term implies that blogs exist together as a connected community (or as a collection of connected communities) or as a social network in which everyday authors can publish their opinions. Since the term has been coined, it has been referenced in a number of media and is also used to refer to the Internet. — Wikipedia
Here are links to pages about closely related subjects.