The command is cp -ax ( see this guide
as an example) . If you want an on-going backup I would suggest rsync. The advantage with rsync is that will update the files when used as opposed to doing the stuf from scratch which can take a long time. If you want further details I will happily oblige. I backup my /home partition hourly using rsync and about once a month do a system backup using PartImage to make an image.