Noun:
A Web site on which an individual or group of users record opinions, information, etc. on a regular basis.
Verb:
Add new material to or regularly update a blog.

Definition

A blog is a discussion or informational site published on the World Wide Web and consisting of discrete entries ("posts") typically displayed in reverse chronological order (the most recent post appears first).

In Simple Words:
Short for Web log, a blog is a Web page that serves as a publicly accessible personal journal for an individual. Typically updated daily, blogs often reflect the personality of the author.

A Blogger is a person who blogs.