In the dim past, computers were so large that they could take up a large amount of space and have their own cooling.  Then in the '70s, there was the minicomputer. This is an Interdata 70, in their later models they changed the color of the rack to dark blue. Then in 1981, IBM changed the game, enter the PC even with very limited memory, sometimes only one floppy disk and the most hated piece of software ever WordStar.   You could get some work done.

In my opinion, the first real SBC was the AMPRO Little Board (1984). It had  Zilog Z80A @ 4MHz, 64Kb of RAM, 1 parallel and 2 serial ports and you talked to it via one of the serial ports.

Enter the SBCs, a plenty:  Intel's Multibus (failure), Motorola's VERSAbus (failure but it survived).

VERSAbus morphed into the VME standard and was later updated to VME64, and of course the most recent to the table is cPCI or Compact PCI, this will stay a long time. And let us not forget the small factor SBC's as well: Rasberry Pi, Beeglebone, TI Launchpads, and it never stops sort of like a cross between an Energizer Bunny, and a Timex watch. So Enjoy, just jump in.