What is a "blog?"

Blog is the contraction universally used for weblog, a type of website where entries are made (such as in a journal or diary), displayed in a reverse chronological order. Blogs often provide commentary or news on a particular subject, such as food, politics, or local news; some function as more personal online diaries.

(Source: Wikipedia- http://en.wikipedia.org/wiki/Blogs)

BlogBib contains more elaborate definitions: http://blog-bib.blogspot.com/