To clone your system to another system.Or make a backup.
In terminal type
dpkg --get-selections | grep -v deinstall > ubuntu-files
this command makes a file list of all installed packages in ur system(and stores it in present working directory).Backup this file in hdd,email, etc...(file is very small)
in the freshly installed ubuntu system
sudo dpkg --set-selections <./ubuntu-files (will set it up and)
apt-get -y update
will install only those packages you had installed (with apt-get) in the older system.
u could back up all the .deb packages from /var/cache/apt/archives/ and install them manually.
dpkg -i *.deb
and after that running an update cycle later.