The Blog Page
The blog page shows all of the "posts" in chronological order. To add to the blog, just write a new post and publish it.
The Blog is just a page that uses the "Blog Page" template. You can use this to make your blog page be named something else. Create a page called "News", "The Latest", etc. and use the template "Blog Page". Make sure to only have one Blog Page though. :)
If you ever delete the blog page (or don't see one now) and want to add it back in, create a new page and give it the page template "Blog Page".
Writing / Editing Posts
This is where you create all the posts for your blog. It operates exactly like any WordPress site.