Making your e-Portfolio "Public" means that anyone who has access to the internet may be able to see it. Search engines will be able to index the pages and they may be included in search results on sites like Google.
The e-Portfolio Directory is a list of e-Portfolios at your school.
Choosing the "Show in Directory" option means that your e-Portfolio will appear in your schools’ e-Portfolio directory for those users who have permission to view your e-Portfolio.
You may choose to make your e-Portfolio "Private to me" and give additional users or a course custom permission to view your e-Portfolio. If you choose the "Show in Directory" only those users who have permission to view your e-Portfolio will see your e-Portfolio in the Directory; other users at your school will not see it in the Directory.
If you choose to make your e-Portfolio "Public within <my school>" and choose "Show in Directory" then your e-Portfolio will be visible in the Directory for all the users at your school.