You have full control over who can write entries on your blog. When a blog is set up, only the blog owner is able to write entries on the blog. However, the blog owner can choose to add 'contributors' - other people with permission to write entries on the owner's blog. There are three different types of contributors:
Authors can write, edit and delete their own entries on the owner's blog, but not edit other contributor's entries.
Moderators can write, edit and delete their own entries on the owner's blog, but also edit and delete other entries.
Administrators have the same privileges as the blog owner, so they can write, edit and delete their own and other entries as well as creating groups, and changing the blog's appearance - everything that a blog owner can do.
For your personal blog, you probably wouldn't want anyone else to write entries on it, but the option is there. Most often, contributors would be added to non-personal or team blogs - when a group of people might all want to write entries on a project blog for example.
Example: You're the head of a research group and own a blog that is for the group. You could add members of the group as 'Authors' so they can add their own entries about the research they're doing.