A blog can be defined as a''Web site containing the writer's or group of writers' own experiences, observations, opinions, etc., and often having images and links to other Web sites'' (http://dictionary.reference.com/browse/blog)
In other words it is a space online where you, the ´blogger´ can publish your opinions and thoughts relating to a given topic. Blogs can be informative or personal. They can be a means of expression and a form of instruction or education depending on the context they are used in.
Here a few examples of some great blogs.
Here is a great mix of blogs for you to get some ideas to start your own blog. Note where they place there advertising.