You should separate a couple of things

Losing data

• You can lose data if you format your drive, or if it breaks.


• Only seperate drives can protect you from hardware failure, partitions don't.


• You can break the filesystem on a partition, but this not very likely. If you do this, only the partition, not the complete disk is affected

Broken OS

If your OS is broken, it can be a really good thing to have your data (like /home, or /movies) on seperate paritions, like you are suggesting. Please do so! This way you can fill up your disc with movies while your OS remains save from having no space at all, and much more of that stuff!

As I read in the comments on one of the other answers, you are doing this. That is good!

Ubuntu file structure

Where windows shows your filesystems with a letter ( Like C: or D:), linux doesn't do this, linux just calls everything a directory.

So you'd have /, and a bit further on you mount the partition home on /home. This looks like it's one big thing, but this doens't mean that if your OS gets 'broken', your /home is gone. You just format your main partition (this will NOT affect your /home. This is a bit tricky, but while it looks like some sort of subdir, it is "mounted", so it's only having some sort of shortcut in that spot. your data is safe!).
After you have choosen your borked partition for a new os, you can choose the partition you use for your home files as mountpoint for the /home directory in your new os, and presto!