The iMac already has a built-in DVD burner which should suit your needs unless you're looking for something that supports LightScribe technology or has a faster burn speed.
That is true but that is too commonly the case with external burners where the software (in this case a copy of a lite version of Nero and the Lightscribe software) is provided for Windows only.
The drives themselves are pretty universal and will normally work fine with Macs, but not the provided software if that is important. Some drives do included versions of software for both Win and Mac with them although in most cases the included software is more of an afterthought (usually limited edition/lite versions of programs) when compared to full featured software like Toast and other packages for Macs.