No operating system/no anything! I have a Samsung Galaxy s3  SGH-I747, I had rooted the phone and had success and tried different ROM's before deciding one I wanted to go back with and stick with.  I had booted my phone into recovery however and left it laying out (ADHD sidetracked me) returned and discovered a friend had accidentally wiped everything- INCLUDING MY BACKUP-he thought I said the backup was on the external sd card, which he noticed wasn't in.  So I have no back-up, no operating system and no firmware because of something with Kies? (it says this on the phone screen???)  which has me completely baffled, because I rooted with Odin.  So, no os, no rom, nothing.........what do I do?
Copy a ROM from your PC to your phone. Flash the ROM in recovery. You may have to reinstall the Samsung drivers on your PC.

Don't worry about this. You still have recovery intact. Keep yourself a SD card with a ROM and GApps on it.

*Always Do a Backup*