Your "homepage" in a traditional html site--the page that shows when you type mydomain.com for example-- would be the page displaying that is in the index.html file.
Joomla! is a database driven CMS so it does not have html pages, but rather pulls up the pieces of pages from a mysql database.
When you install Joomla! by default it has a menu link to the frontpage component as the home page. However, any content or component or other link can be used as the "home" page. The default page is controlled using the Main Menu.
Changing the default page.
The "page" that shows is determined by the Main Menu. Go to Menu ==>Main Menu. Select the item you would like as the homepage and click the default icon.