Here is how Wikipedia explains what blog is.
A weblog (usually shortened to blog, but occasionally spelled web log) is both a web application used for entering, modifying and displaying periodic posts (normally in reverse chronological order) as well as the totality of content constituted by these posts. Initially, weblog content was manually written on personal web pages, but soon after their appearance, weblogs began to be implemented from web-based software, and this is the norm today.
To me, it isn't a new fashion. Many people take it as a new way of personal publishing on the Web, writing and recording this and that. Have you been to BBS個人版? It is like an extension of that. On the other hand, it is just like what Wikipedia says, a web log. Log is a time-sequential data record. Thus, it is not surprised that it looks like a diary.
So, why do so many people take a fancy to blog? People dislike being longly and tend to show themselves somehow. They have thier own webpages and post essays and talk on forums and BBS. Now, blog is a easier and simpler way of having a personal features 'garden' to speak out.
That's it. Not a innovative idea. You can say it is a evolution version of personal web page. Like it or not, it is everywhere on the big ocean of Internet.