Modifying Roles

Can you modify roles? Looking at the chart of roles the “author” is the lowest role that allows posting. This also means that they can edit so does that mean that they can edit their posts or everyones? I’m assuming everyones because the role below “author” is “contributor” and they can edit but not post. Seems a bit odd. I would like for someone to be able to post but only edit their own post or not at all.