Expanded memory was a system where you could set a window under the 1Mb mark into the expanded memory bank. The window could be moved to expose a different section of the memory. It was developed to help store additional information (particularly spreadsheets) that just couldn't be squeezed in. There was no virtual memory on Intel processors at this time.
Extended memory was simply memory located above the 1Mb mark. This was only available on 286 processors or above, as previous processors only had a 20 bit address bus. (The 286 has a 24 bit bus, and the 386 a 32 bit one). Unfortunately, MS-DOS, the operating system of the time, did not allow the processor to be in the mode required to access this memory, so various memory extenders were required to switch the processor into and out of the mode as required. (HIMEM.SYS later helping out with this task).
Expanded memory come first, when pre-286 processors were common and the
640kB limit at its worst. Extended memory took over by the time 386s came along with their fast context switches (and Windows 386 Enhanced and DOS4GW). It is far easier to work with and much less limited in size and performance (and does not require additional hardware).Christian.