Like the others have said, the ROM3 boards are a complete redesign of the system, like a Mac Plus or SE is to a Mac 512Ke.

The ROM3 allows the use the GS-OS which is the Apple II version of Mac System, with windows and other nice things. Its basically becomes a 6502 Mac - LOL! Before this, GEM (used on the Atari ST) was developed for many machines including Apple IIs but Xerox failed to properly complete with Microsoft and basically lost the Digital Wars. And C64's GEOS was also later released for the Apple II as well.

Enough of my mindless rants...