The key difference between ENIAC and EDVAC lies in how they stored and executed instructions. ENIAC had a fixed program design, meaning the instructions were wired into the machine and could not be changed easily. EDVAC, on the other hand, introduced the concept of a stored-program architecture. Instructions were stored in memory alongside allowing for greater flexibility and easier modification of programs.

This innovation paved the way for modern computers with stored programs on hard drives or other storage devices.