Sitecore stores content used in your website in multiple databases. When you start developing in Sitecore it is really important to understand what content is in which database.
The master database is where content is authored. It contains every version of every item.
The core database stores settings that control how Sitecore functions. Adding “programs” to the Sitecore menu or commands to ribbons in page editor is done through the core database. The core database also holds the login credentials for the sitecore and extranet domain users.
The web database has the currently published version item from the master database. This is the content that is displayed for public users.