To backup your Ubuntu install you don’t just need to keep a copy of the
/home folder (or partition), since you would still need to re-fetch all the packages you installed, and that can be time consuming, especially if you carefully chose the applications to add to the system.
Synaptic already offers a similar function, which is File > Save Markings As… (be sure to fill the check box “Save full state, not only changes”, otherwise you will be probably getting a 0byte file). You can then use the File > Read Markings function to restore the package list on another system/install.
What’s the deal with this? The function actually saves indiscriminately a list of all the installed packages, including those that were installed just because they were a dependence. For example if you sudo apt-get/aptitude install packagename you will probably install also packagename-data and packagename-core or something along those lines, as they are dependencies of packagename, but the dependencies may be more complex and deeper (for example, packagename-core may also require other packages in turn); dependencies can change over time, so if package A requires package B today, a month from now that may be not true anymore; so if you passively restore the whole packagelist, you would be installing package B even if that’s not needed anymore.
The solution is to save a list of only the “main” packages, which will in turn require the correct dependencies; this can be achieved with:
aptitude search "~i ?not(~M)" -F "%p install" > packagelist.txt
This saves into packagelist.txt the list of the installed packages (~i) that were not installed automatically (not(~M)), mantaining the same format of the list generated by Synaptic, that is “packagename install” in each row, so you can seamlessly import it from Synpatic.