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When working with memory within DOS or Windows 9.x the following terms and concepts about the memory model need to be kept in mind.

Conventional Memory - The first 640K of memory addresses used to run applications. Also referred to as Lower Memory.

Reserved Memory - The next 384K of memory address reserved for use by different types of ROM BIOS and Video RAM. Also referred to as Upper Memory.

Extended Memory (XMS) is any memory address above Reserved Memory, (above 1024K).

Himem.sys is an Extended memory manager (XMS) that coordinates the use of extended memory, including the high memory area. This prevents two applications or device drivers from accessing this area of memory simultaneously.
The device= statement is required in the config.sys for Himem.sys to load properly.

device=c:\windows\HIMEM.SYS
devicehigh= c:\windows\HIMEM.SYS

The HIMEM.SYS command line must come before any commands that start applications or device drivers that use extended memory.
Himem.sys must be loaded first in Config.sys (Starting with Windows 95 Himem.sys was incorporated into io.sys.)

High Memory Area (HMA) is the first 64K of Extended Memory.

Expanded Memory (EMS) is Reserved or Extended Memory which is made to act like Conventional Memory. Also known as LIM Memory. (LIM – Lotus, Intel, Microsoft)

Limulation is the act of converting Extended Memory into Expanded Memory.

Emm386.exe uses extended memory to simulate expanded memory for programs that can use expanded memory and also makes it possible for programs and device drivers to be loaded into UMBs (Upper memory blocks). Himem.sys provides the means to access UMBs (opens the door) and Emm386.exe is the tool that accesses UMBs.

UMBs (Upper Memory Blocks) are unused memory addresses in the Reserved Memory area.