A blog is a discussion or informational site published on the World Wide Web and consisting of discrete entries ("posts") typically displayed in reverse chronological order. Although not a requirement, most good quality blogs are interactive, allowing visitors to leave comments and even message each other via GUI widgets on the blogs, and it is this interactivity that distinguishes them from other static website In that sense, blogging can be seen as a form of social networking. Indeed, bloggers do not only produce content to post on their blogs, but also build social relations with their readers and other bloggers.
Most blogs are primarily textual, although some focus on art (art blogs), photographs (photoblogs), videos (video blogs or "vlogs"), music (MP3 blogs), and audio (podcasts). In education, blogs can be used as instructional resources. These blogs are referred to as edublogs.
- Personal blogs - The personal blog, an ongoing diary or commentary by an individual, is the traditional, most common blog. Some sites, such as Twitter, allow bloggers to share thoughts and feelings instantaneously with friends and family, and are much faster than emailing or writing.
- Microblogging - Microblogging is the practice of posting small pieces of digital content—which could be text, pictures, links, short videos, or other media—on the Internet. Microblogging offers a portable communication mode that feels organic and spontaneous to many and has captured the public imagination.
- Corporate and organizational blogs - A blog can be private, as in most cases, or it can be for business purposes. Blogs used internally to enhance the communication and culture in a corporation or externally for marketing, branding or public relations purposes are called corporate blogs.
- By genre - Some blogs focus on a particular subject, such as political blogs, health blogs, travel blogs (also known as travelogs), gardening blogs, house blogs, fashion blogs, project blogs,education blogs, niche blogs, classical music blogs, quizzing blogs and legal blogs (often referred to as a blawgs) or dreamlogs. While not a legitimate type of blog, one used for the sole purpose of spamming is known as a Splog.
- By media type - A blog comprising videos is called a vlog, one comprising links is called a linklog, a site containing a portfolio of sketches is called a sketchblog or one comprising photos is called aphotoblog. Blogs with shorter posts and mixed media types are called tumblelogs. Blogs that are written on typewriters and then scanned are called typecast or typecast blogs; seetypecasting (blogging). A rare type of blog hosted on the Gopher Protocol is known as a Phlog.
- By device - Blogs can also be defined by which type of device is used to compose it. A blog written by a mobile device like a mobile phone or PDA could be called a moblog. A practice of semi-automated blogging with live video together with text is referred to as sousveillance. Such journals have been used as evidence in legal matters.
- Reverse blog - Reverse Blog is composed by its users rather than a single blogger. This system has the characteristics of a blog, and the writing of several authors. These can be written by several contributing authors on a topic, or opened up for anyone to write. There is typically some limit to the number of entries to keep it from operating like a Web Forum.