Debian Installer, also known as «d-i», is the software system to install a basic working Debian system. A wide range of hardware such as embedded devices, laptops, desktops and server machines is supported and a large set of free software for many purposes is offered.
The installation is conducted by answering a basic set of questions. Also available are an expert mode that allows to control every aspect of the installation and an advanced feature to perform automated installations. The installed system can be used as is or further customized. The installation can be performed from a multitude of sources: USB, CD/DVD/Blu-Ray or the network. The installer supports localized installations in more than 80 languages.
The installer has its origin in the boot-floppies project, and it was first mentioned by Joey Hess in 2000. Since then the installation system has been continuously developed by volunteers improving and adding more features.
More information can be found on the Debian Installer page, on the Wiki and on the debian-boot mailing list.