It is OK to have your system and home on the same drive (with or without separate partitions). The important thing is to have the backup somewhere else: on an external drive, in another house, in a safe-deposit box ...Thanks to kjohri for the tutuorial link. Most helpful.
As to the questions. Yes 50 Gig. And yes lots of programmes ... but also images, sound and video files. All ubuntu is in one partition. So I guess it is a home folder in the root partition.
However something else has come up that I don't quite understand. Is not a bunch of files on an external hard drive equivalent (or indeed better) than them being in a separate partition on the same drive as the o/s? Sort of like a belt and suspenders ...
Once I get it figured out how to 'rsync' I'll see what I can do about taking your advice on the various backups. Thanks for sharing the expertise.