A blog is a website for which an individual or a group frequently generates text, photographs, video, audio files, and/or links, typically (but not always) on a daily basis. The term is a shortened form of weblog. Authoring a blog, maintaining a blog or adding an article to an existing blog is called "blogging". Individual articles on a blog are called "blog posts," "posts," or "entries". The person who posts these entries is called a "blogger".
Want to know some interesting history of evolution of blogging:
The term "weblog" was coined by Jorn Barger on 17 December 1997. The short form, "blog," was coined by Peter Merholz. He broke the word weblog into the phrase "we blog" in the sidebar of his weblog in April or May of 1999.  "Blog" was accepted as a noun (weblog shortened) and as a verb ("to blog," meaning "to edit one's weblog or a post to one's weblog"). 
Justin Hall, who began eleven years of personal blogging in 1994 while a student at Swarthmore College, is generally recognized as one of the earliest bloggers. The site Xanga, launched in 1996, had only 100 diaries by 1997, and over 50 000 000 as of December 2005. Blog usage spread during 1999, being further popularized by the near-simultaneous arrival of the first hosted blog tools: Evan Williams and Meg Hourihan (Pyra Labs) launched Blogger.com (which was purchased by Google in February 2003), and Paul Kedrosky's started GrokSoup. As of March 2003, the Oxford English Dictionary included the terms weblog, weblogging and weblogger in their dictionary. 
Dave Winer is the one of the pioneers of the tools that make blogs more than merely websites. One of his most significant contributions was setting up servers that weblogs could ping to update themselves.
Blogging combined the personal web page with tools to make linking to other pages easier - specifically blogrolls and TrackBacks. This enabled bloggers to control the threads that connected them to others with similar interests, thereby wresting control from forum moderators