Almost every application or dynamic website requires a backend to manage it.
These admin screens come bundled with ready-to-use apps (like WordPress, Magento, etc.) but custom apps need custom backends.
It is usually considered as a simple process to create an admin template but, most of the time, this requires a lot of effort as there are lots of things (usually related to usability) to think of.
Designers/developers out there have created various free admin templates and here they are so you don't need to search:
Charisma is a responsive admin template that is built with Twitter Bootstrap.
It has 9 skins and includes almost everything we may need like forms, charts, buttons, tables, file manager, image gallery and much more.
Here is also the GitHub link for the template in case you want to fork it.
INADMIN is a jQuery-powered admin template with header + top navigation and styles for tables, forms, messages and more.
It also includes a nice sign-in page.