Before there was Windows and it’s ability to emulate multitasking in a single tasking graphical environment there was DOS. Dos did not multitask. The minds at Quarterdeck created a solution for true multitasking in a DOS environment and called it DESQview.
Wikipedia has some pretty good information on DESQview.
HERE <-Clicky the link to see.
Some knowledge of DOS and memory management software is needed. Back in the scene this was used to run multiple line BBSes in DOS.
The original DESQview was succeeded by DESQview/x .