Q: burning disks on iMac without a disk drive

Buy this, works great.

I use it with iMovie 06 and iDVD 09.  Using Mac Pro and Mac Mini.

http://eshop.macsales.com/item/Other%20World%20Computing/MRSFW8U3/

There are hundreds of external optical drives. Starting with Apple's own Super Drive.

http://store.apple.com/us/product/MD564ZM/A/apple-usb-superdrive

Most if not all external optical drives should work directly.

http://eshop.macsales.com/shop/firewire/optical-drives/

I would go for USB personally over Firewire.  With that said, the Apple branded one should work just fine with iDVD. And you have a 14 day no questions asked return period should it somehow not work correctly.

I recommend you get a 3rd party optical drive with drawer loading.  The slot loading drives like the Apple drive are hard to clear if there's a stuck disk.  Also the Apple drive only supports USB 2.  The drive that Ziatron recommended supports USB 3. That's the same one I got to use with FW/Thunderbolt because my iMac doesn't have USB 3 and I needed faster read performance.

Because they are optical drives, unlike magnetic drives which are the majority.

Don't get Firewire.

Connecting via FireWire, which is slower, less widely supported and more expensive than USB3.

And, as the maximum speed from a CD/DVD drive is less than 300MB/s it would be no slower if it were on a USB2 bus either.

Firewire is an older interface. It has been phased out of the majority of Apple products, and as such you would need an extra adapter to go from Thunderbolt port to Firewire, or USB to Firewire.  Its better to just go USB from the get go.

USB 2 more than covers optical drive read / write speeds.

Its optical, because it uses a type of laser "eye" or lense (optic) to read and write discs.  Instead of magnetic interface of other disk drives. It is in fact quite different to your standard hard drive, mechanical or ssd.

Tray loaded external optical drives, tend to be much bulkier than slot loaders.

Yes it easier to get a stuck disk out of a tray loader.