If you are working on your site and you want to put your site offline for a while, then you will have noticed you get the default Joomla offline page. This is a white page which contains the Joomla logo, your offline message and a login form. Probably this is not what you would like. Not because you don't like the Joomla logo, but this may confuse the visitors of your site. You want your visitors to see clearly that they are on the right place and should come back later. If they see a logo they don't know, they might think the site is gone. So you want to have your own page with your own logo and probably you want to get rid of the login form as well. In short, you would like to have a customized offline page for your site. Well, here's the way to do this.
You can find the default Joomla offline page in /templates/system/offline.php. You can change this file to have your custom page, but that wouldn't be very smart. If you change this file, then there's a good chance your changes are overwritten when you upgrade to a newer Joomla version. Just copy this file to the root directory of your template (=/templates/yourtemplate/). Now you can start editing this file to an offline page that fits your site. As a start I would recommend to include the css file of your template. And what else you need to change, depends on what you would like to have on this page. With a little bit of html knowledge you can make the perfect offline page for your Joomla site.
One quick way is to replace the /images/joomla_logo_black.jpg with your own logo or message, as this is where the 'offline' page get the Joomla! logo from. Of course, you will have to name your own file joomla_logo_black.jpg. Move the original to another location and then put your own version in the above path.