what hardware is needed for multiple-screen displays

I just saw an ad for an ASUS monitor showing 9 monitors in a 3x3 matrix
being used to show a picture. It got me curious.

Other than the monitors themselves, what hardware would be needed to do
that? For example, would something like that need 9 graphics cards,
one for each display? Instead, is that done using a specialized card?
Does it require custom software? Does it require high-end CPUs and/or
massive amounts of RAM? Or are there specially made electronic devices
produced to do that?

Thanks

John