Normally.. ROM includes the entire system, Kernel, system files, etc. But in the case of the DroidX, we can't change the kernel or boot img, etc. due to the locked bootloader.
So, a ROM for the DroidX is the closest we can come to a true ROM.. It replaces core system files.
A Theme changes a few framework files, and often several application files (by change, I mean replaces), with ones that are themed.
So, if you are looking to improve system performance, find different functionality, etc. You want a ROM. If you want it to be pretty, then you are looking for a theme.
On another note: Themes replace some of the framework/app files. So, if you're using a ROM, you want to make certain that the theme was created using the ROM you want as it's base.
