DESQview was released in July 1985 (the original version), four months before Microsoft introduced the first version of Windows. It was widely thought to be the first program to bring multitasking and windowing capabilities to DOS, but in fact there was a predecessor, IBM's failed TopView, released in 1984, from which DESQview inherited the popup menu.
Under DESQview, well-behaved DOS programs could be run concurrently in resizable, overlapping windows (something the first version of Windows could not do). A simple hidable menu allowed cutting and pasting between programs. DESQview provided for simple editable macros as well. Quarterdeck also developed a set of optional utilities for DESQview, including a notepad and dialer. Later versions allowed graphics mode programs to be loaded as well, but only run in full screen mode.
DESQview was not a full-fledged GUI operating system; it was a quasi-GUI shell that ran in real mode on top of DOS. Although it could be configured to run on an Intel 80286-based PC AT with two megabytes of memory, it really came into its own on Intel 80386 machines which were better at utilizing memory above DOS's limit of 640 KB. However, in either case, it ran in real mode rather than protected mode, meaning that a misbehaving program could still crash the system.