Gaining popularity in the past half decade, some sources claim that between 2002 and 2006 there was almost a 400% increase in the number of blogs created per month. (http://www.caslon.com.au/weblogprofile1.htm )
Blogs are essentially web journals. The etymology of the word, ‘blog’ is ‘web-log.’ Weblogs in more technical terms are, “a hierarchy of text, images, media object and data, arranged chronologically, that can be viewed in an HTML browser.” (Berkman Center for Internet & Society at Harvard Law School )
Blogs can include anything that you want it to, and themes for blogs vary from photography to corporate. Within the last couple of years corporate blogs have gained popularity.
