I have dedicated this post to explain what is a blog...

A blog is simply an online diary. People use blogs to communicate.

Its almost similiar to a travel log. Ever thought of what old adventurers or sailors did so that they would not get lost? They described their journeys in diaries or journals so that they could refer to it at a later time. These documents were passed on to later generations; it served them as a guide for their journeys and thus we have people who discovered new lands etc. In fact this was how Columbus discovered America.

A blog may be used as a travel log too, but there are various other uses also. Scientists use blog to communicate ideas realted to their research; a manager of a firm might use a blog to describe how well his firm was doing over the years, etc. The basic idea is communication.

But a blog need not be something that is always public. You can use it like a private diary if you like...