ISPF (Interactive System Productivity Facility) is the user interface and supporting programs that come with IBM's OS/390 operating system and that allow a company to configure and manage its system, add new system or application programs and test them, and access system files (data sets). ISPF consists of:
IBM also offers an ISPF that will run on an OS/2 system so that developers can work "offline" from the OS/390.
For those not familiar with the way of life on IBM mainframes, it should be noted that the "user interface" mentioned is a mask oriented, 25*80 text-only interface with a few special graphics characters thrown in for drawing boxes around things and such. ISPF offers the user a system of deeply nested menus and a set of programs that do what the menus indicate. Sounds like nothing much, but it's still the bread and butter working environment for many well-paid IBM developers.
Some people equate ISPF with the (again, very mask/screen/menu oriented) text editor in ISPF. For an editor that "interacts" only when the user hits ENTER to fire off an update, and for working on mostly fixed-column source code, ISPF's editor is remarkably powerful and effective.