is an Inno Setup extension. It provides possibility to change the graphic look of whole installer - backgrounds, icons, even Next/Back/Cancel buttons.

To create a Inno Setup installer, you first have to write a Inno Setup script which is ASCII text file with a format somewhat similar to .INI file. You can edit scripts with every text editor. It's recommended you use a text editor that shows line numbers because Inno Setup uses line numbers to indicate where errors lie, and to warn you about where errors might lie. An editor that supports syntax highlighting is also recommended.
At this moment several editors (IDEs) exists. You can choose one which fits all your needs. For more details visit Inno Setup Third-Party Files.

Scripts have an ".iss" (meaning Inno Setup Script) extension. The script controls every aspect of the installation. It specifies which files are to be installed and where, what shortcuts are to be created and what they are to be named, and so on.
Inno Setup Scripts are arranged into sections. Each section controls a different aspect of the installation. A section is started by specifying the name of the section enclosed in square [brackets]. Inside each section is any number of entries.
There are two different main types of sections: those such as [Setup] whose entries contain directive names and values (in the form Directive=Value), and those such as [Files] whose entries are divided into parameters. For other information about Inno Setup script files visit Inno Setup documentation.

Each Graphical Installer powered installer epitomizes more sophisticated system. It includes several other Inno Setup headers and uses a few plug-ins to create required visual effects.

Installer is saved in it's own .iss file called <project>.iss file where project represents installer's filename. This is main installer's script - this file is compiled into resulting .exe. This file contains link to all necessary files and this file is a heart of your future installer.
Graphical Installer powered installers are unique in fact that they need aditional file called <project>.graphics.iss where resources for proper function of installer are defined.

With this two files files you are able to create skinned installers with all functionality as classic Inno Setup installers.

Please read <project>.iss description and <project>.graphics.iss description for more information.