A blog is a regularly updated, web publication of personal thoughts and related links.
So basically it is nothing more than a website. However, the blog is usually ordered by date rather than subject and acts like a diary.
According to google it is (usually) a free, automated web publishing tool that sends updates to a site.
People have been writing web-logs for quite a while, but the trend only took off with the introduction of automated published systems, such as blogger.com. and blogspot.com These services allowed people with no web publishing expertese to easily publish their blogs on website for little or no cost.
Blogs are being written about absolutely everything. Too keep up to date on what is beeing written, visit technorati.