Android is a form of Linux that runs on phones & tablet. Obtaining "root" access allows you to get full permission to the hardware/software of your device. "ROMS" are software packages you use to overwrite the default Android version your phone or tablet vendor gave you. This can be helpful since some vendors use outdated versions of Android OS, put hidden background tasks on the phone or lock you out from certain features. I do not have an Android phone tho...
I have a G2 with the cyanogen Mod. It's the best phone I've owned. Or are you talking about game roms???
I got a Thunderbolt, its my first Android phone and I love it. I've been planning on eventually rooting it but just thought I'd stick with the default setup at first. A recent HTC update has started giving me some random reboots making me think of trying a custom ROM that might be more stable. I'd want to make sure I can reload the original OS if I need to take this think back to Verizon. Root would be cool if I could get rid of some of this bloatware and make the thing a bit zippier. I'm not sure exactly what other benefits I'd use from root though.
my EVO is rooted and equipped with the plain and simple sprint lover's rom. However, I have to unroot because my phone is messing up and I have to bring it in for service.