Two types of memory can be added to a PC to increase memory beyond 1MB: expanded memory and extended memory. Expanded memory conforms to a published standard called EMS that enables DOS programs to take advantage of it. Extended memory, on the other hand, is not configured in any special manner and is therefore unavailable to most DOS programs. However, MS-Windows and OS/2 can use extended memory.

DOS Protected Mode Interface (DPMI)
This is a programmer's reference copy of the DOS Protected Mode Interface, a protected mode API specification for DOS extended applications.

Overview of memory management
This page, from Qualitas Inc., provides a detailed explanation of the 8088 CPU, low/high DOS, expanded and extended memory, and 386 capabilities.