No Really, What's a Blog?

What is a blog? A type of website or part of a website supposed to be updated with new content from time to time. But I don't get it... what is the point? I always thought a blog was a place for people to rant about their awful boss, or how annoying that girl is at school... are they supposed to be informative?
A blog can function as many different things, yes, it can be a place for people to rant about useless things not interesting truely anyone on the interent in which case it is refered to as a personal journal of posts. (Yes, a diary).
A blog can be used by a company to spread awareness and advertise throughout the web, in which case their posts would consist of company deals, coupons, are exciting new products!
A blogger could also dedicate their entire blog to art! All of which was produced by them, to spread awareness and gain more customers interested in their art. Or an entire blog dedicated to acknowledging other's art, such as the website Tumblr, in which bloggers can reblog interesting photos, or sayings they find scrolling through their dashboard.
But after reading through articles, and blog posts, as to what a blog is.. I started to ask myself interesting questions. Is my Twitter account a blog? And the answer is yes! A Twitter, and a facebook for that matter is considered a blog! Think about it, you can follow companies on twitter, or like products on facebook- an advertisement blog. You can follow/like your favorite celebrity on both sites in which you can read their personal journal enteries throughout their day.
No, I did not know or understand the purpose of a blog untill I realized the sites that I was using every day were blogs! It is interesting to think of how much technology has changed and how you can pretty much express yourself, sell things, and admire things anywhere!