The three main purposes of writing are:
To inform
When you are writing to inform, you are presenting information that the reader is interested in. Writing purely for the sake of information should avoid bias and be objective.
To persuade:
Persuading writing usually tries to convince the reader of a fact or point of view. This kind of writing can include reasons that support an idea, or rather information presented with bias to convince the reader of the writer's view.
To entertain:
Writing designed to entertain creates and emotion in the reader. Entertaining writing is usually funny, interesting, or even sad. It is created so that the reader will enjoy what is written.
Links: Check out these sites and decide the purpose of each:
http://www.bloglockers.com/
http://engadget.com/
http://lifehacker.com/
http://gizmodo.com/
http://kimjongillookingatthings.tumblr.com/
http://opinionator.blogs.nytimes.com/
http://popeater.com/
http://redstate.com/
http://www.slashfilm.com/
http://www.eurogamer.net/
http://www.michaelmoore.com/blogger/mmflint/
http://weblog.greenpeace.org/
http://www.1000awesomethings.com/