Remastersys is a tool that can be used to do 2 things with an existing Debian, Ubuntu or derivative installation.
1. It can make a full system backup including personal data to a live cd or dvd that you can use anywhere and install.
2. It can make a distributable copy you can share with friends. This will not have any of your personal user data in it.
It was initially created to be able to easily backup or create a distributable copy of an Ubuntu or derivative installation. Inspiration for this tool came from the mklivecd script that Mandriva uses and the remasterme script that is in PCLinuxOS. Since those scripts were not very easy to port to Ubuntu, it was written from scratch.
It is a very useful and easy way to create a customized Live CD/DVD version of Ubuntu. The resulting iso can also be installed on a USB pendrive, creating a Live USB distro, using either a command-line approach or a graphical tool such as UNetbootin.
It has a command line version and a GUI version. It currently works with Ubuntu, and possibly more Ubuntu-based distributions.