It's whatever you want it to be
- a list of specials
- your latest news
- a personal/business diary
- a collection of links
- memo to the world
A blog is a web site, where you write stuff on an ongoing basis. New stuff shows up at the top, so your visitors can read what's new. Then they comment on it or link to it or email you. Or not.
Our blog is with blogger.com and we take an RSS feed from it to display on our web site.