A blog (a contraction of the term "Web log") is a Web site, 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. "Blog" can also be used as a verb, meaning to maintain or add content to a blog.(Source: http://en.wikipedia.org/wiki/Blog)
Many blogs provide commentary or news 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.
The ability for readers to leave comments in an interactive format is an important part of many blogs. Most blogs are primarily textual, although some focus on art (artlog), photographs (photoblog), sketches (sketchblog), videos (vlog), music (MP3 blog), audio (podcasting), which are part of a wider network of social media.
Micro-blogging is another type of blogging, one which consists of blogs with very short posts. As of December 2007, blog search engine Technorati was tracking more than 112 million blogs. With the advent of video blogging, the word blog has taken on an even looser meaning — that of any bit of media wherein the subject expresses his opinion or simply talks about something.
There are many different types of blogs, differing not only in the type of content, but also in the way that content is delivered or written.
The personal blog, an ongoing diary or commentary by an individual, is the traditional, most common blog. Personal bloggers usually take pride in their blog posts, even if their blog is never read by anyone but them. Blogs often become more than a way to just communicate; they become a way to reflect on life or works of art. Blogging can have a sentimental quality. Few personal blogs rise to fame and the mainstream, but some personal blogs quickly garner an extensive following. A type of personal blog is referred to as "microblogging," which is extremely detailed blogging as it seeks to capture a moment in time. Sites, such as Twitter, allow bloggers to share thoughts and feelings instantaneously with friends and family and is much faster than e-mailing or writing. This form of social media lends to an online generation already too busy to keep in touch.
A blog can be private, as in most cases, or it can be for business purposes. Blogs, either used internally to enhance the communication and culture in a corporation or externally for marketing, branding or public relations purposes are called corporate blogs.
Is a type of blog that answers questions. Questions can be submitted in the form of a submittal form, or through email or other means such as telephone or VOIP. Qlogs can be used to display shownotes from podcasts or the means of conveying information through the internet. Many question logs use syndication such as RSS as a means of conveying answers to questions.
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 a photoblog. Blogs with shorter posts and mixed media types are called tumblelogs. A rare type of blog hosted on the Gopher Protocol is known as a Phlog.
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. One early blog was Wearable Wireless Webcam, an online shared diary of a person's personal life combining text, video, and pictures transmitted live from a wearable computer and EyeTap device to a web site. This practice of semi-automated blogging with live video together with text was referred to as sousveillance. Such journals have been used as evidence in legal matters.
Some blogs focus on a particular subject, such as political blogs, travel 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.