Does restoring a TWRP backup restore everything, including the ROM? Suppose I were to do the following:


Make a backup with TeamWin Recovery (TWRP)
Delete everything (not the backup of course) including my ROM and all
Flash a new ROM, then after having fun with all that, delete everything once more
Restore the previous backup via TWRP


Would my ROM, my apps, and my data, all be restored to their original state?
Yes, your earlier ROM will be fully restored as long as you backup system partition as well since that's where the ROM resides. If you wants your apps and apps' data, then you should back up data partition.

Note: Backup feature doesn't backup SDcard's content. If you delete all the data of the SDcard you will not be able to restore it.