System Images are also backups. A system image is an entire copy of your system as it is at the time you created the image, restoring that image will bring you back to the point your computer was when you made the image, a complete mirror image. Any documents etc made after that will not be included if they were saved to the drive, after you made an image of said drive.
In reference to your question It would depend on the software/company, if you mention which software/company people may understand better what they may mean.
A system restore point, will restore the computer back a few days, weeks etc, and eliminate certain changes you have made since creating a restore point. However, any documents etc you created and saved onto that drive will remain intact.
For instance, creating a system restore point before drivers are updated will enable you to go back to before you installed those new drivers if there is an issue with them. System specific files will be backdated.
This part I am not too clear on -
There are also basic document backups, a backup of your personal data, data you have stored in your "My Documents" folder etc, and user specific settings, I think, like your libraries, desktop, contacts and favorites. I don't make them so I am not sure. This backup can be found by pressing Winkey + S and searching for "File History" (If I am correct)
I hope I didn't confuse the issue.