Hello carlito777 and welcome to BC!
Rather than me trying to explain it, I'll just quote the page on the Microsoft site:
System Restore can make changes to Windows system files, registry settings, and programs installed on your computer. It also can make changes to scripts, batch files, and other types of executable files on your computer. Personal files, such as documents, e‑mail, photos, and music files, are not changed.
When creating a system restore point, a full registry backup is made, as well as backing up installed programs etc. (the items listed above)
Backing up your registry (such as with ERUNT) will only backup your registry, nothing else.
I would recommend you make a full backup of your registry before making any changes to it, even if you're experienced with it, things can still go wrong, potentially rendering your machine useless. But apart from that, a system restore point would be a better thing to do. It covers more files/locations so can correct a wider range of errors
Restore points are made automatically when installing certain updates and software, but if you're doing something risky (resource editing etc.), then it would be a good idea to make one manually.