Definition of blog

A webpage containing information about someone's activities or interests. People who read a blog can add their own opinion about what it contains. [1]

A blog or weblog is a content management system that allows a user with no technical knowledge to create a dynamic webpage, typically with a chronological structure.  Usually, readers can add comments and blogs are characterised by so-called “permalinks” - these are permanent links that maintain consistency as they move from the front page to the archives.

Blogs generate a loudspeaker effect on the web, since they get indexed by search engines in an efficient way.  Usually blogs are frequently updated and have a high link density since it is customary to link to the sources used in the text.  Also, they are written in clean code as the code is generated automatically, thus optimising their visibility.

Example
Blogs became immensely popular in 2003 when Google bought Blogger, a blogging service created by the startup Pyra Labs, the ease of use determined a huge growth that was crucial to the development of the so called “conversational web”, “social web” or “web 2.0”. [2]