What determines your home page?
Your "home" page in a traditional HTML site--the page that shows when you type mydomain.com for example--is the page in the index.html file.
Joomla! is a database-driven content management system (CMS); it does not have HTML pages, but rather assembles the pages from the MySQL database.
When you install Joomla! by default it has a menu link to the frontpage component as the home page. However, any content, component or other link can be used as the home page.
Changing the Home Page
The "page" that shows is determined by the Main Menu. Go to Menu > Main Menu. Select the item you would like as the home page and click the default icon.