Blog is short for Weblog.
A blog is a website, but it's content is much more informal and free-flowing than you will find on a business website. A blog is also typically updated much more frequently than a company's website, sometimes as often as several times a day.
When you visit a blog, your first impression is that it is an online diary or journal because that is exactly the way it is organized. Each entry or post is displayed in reverse chonological order with the newest at the top.
It is important to understand that while your blog could be your company's website, the more typical scenario is that your blog is seperate and distict from your official company website and each has its own URL. The usual arrangement is that the content on your company website is pretty static and plays the role of your company's official brochure. Your blog, on the other hand, has a much more informal tone and can be used to educate and to expound on topics in your industry that you feel passionate about - your soapbox, if you will.
An additional thing that makes blogs so attractive is that you don't have to wait for your webmaster to update it for you. The blogging software is easy to use and it takes no time at all for you to bang out a few sentences and publish them for the world to read.