Comparing a blog to a ship log kept by sailors 200 years back Andrew Sullivan writes in the The Atlantic:As you read a log, you have the curious sense of moving backward in time as you move forward in pages—the opposite of a book. As you piece together a narrative that was never intended as one, it seems—and is—more truthful. Logs, in this sense, were a form of human self-correction. They amended for hindsight, for the ways in which human beings order and tidy and construct the story of their lives as they look back on them. Logs require a letting-go of narrative because they do not allow for a knowledge of the ending. So they have plot as well as dramatic irony—the reader will know the ending before the writer did.
Most ship logs were written as records but some were written (and edited during publication) to impress the public. However in the case of blogs the reverse is true. Most of them are written to express one's point of view and elicit a response. We always wait to hear from our readers.
If you ever have a chance to read postings on a blog from the most recent to the first, you would probably discover the journey of the blog. In that sense it is more like a diary where the writer does not know his destination and is merely writing as he feels. Many times the blogger may even contradict himself over a year or two. This can also happen in the case of a diary.
However there is one crucial difference. You can point to writer his failings in a blog. In the case of the diary its better to return it back to its secret place and pray that the topic never comes up...