Well, the names are kinda self explanatory, but this what they do:
- Create a system image: A system image is a complete copy of your computer in its current state. This includes all of your files, and installed programs. If you ever restore from a system image in the future, it'll be exactly like it was when you created the image.
Note that this can be stored anywhere, but you cannot boot from it, ie, you need a working Windows installation to restore your PC to an earlier state using an image. Alternatively, if your computer came with a recovery partition, you can boot into it, and then restore your PC using an image.
Create a system repair disk: A system repair disk will try to, well, repair your OS (ie, system files used by Windows) in case it is not working properly. It will NOT, however, do anything to repair/restore your personal data, or changes that you've made to the OS, such as installing software. Also note that it cannot be used to re-install Windows.
Create a recovery drive: This will allow you to completely reset your Windows installation in case something goes wrong with your installation, but will not recover any of your personal files. If your computer came with a recovery partition, this may not be required.
In your case, since you may completely remove the Windows OS while installing Ubuntu, you should create a recovery drive. While you can also use create a system image or a system restore point and then restore your PC to it via the recovery partition, some PCs come without a recovery partition, and chances are that while installing a new OS, this partition may get corrupt too.
The reason you cannot use a DVD drive to create a recovery drive is that these are usually too large to fit on DVD drives. Flash drives aren't too expensive nowadays, and you can use a 8GB+ one perfectly, especially since your OS is a new install and doesn't have too many files/extra software.
Pro tip: Take note of your Windows 10 serial key too, and save it somewhere. In case you lose your Windows 10 installation, you can create a installation drive on another PC, and then use it to install Windows on your computer with the serial key.