What's a blog?

So it's my first time blogging. Never been 100% open to the idea of blogging, but not 100% closed minded to the idea. I guess having a blog could potentially be a way to meet people, or it could expose a lot about you. My reason as to making a blog is that I will receive extra credit for my English class, and it is a good way to be updated on class assignments and talk with classmates, via my blog or my teachers blog.