Frequently Used Terms :

An exe file consists of alotof characters / numbers. 

These are interpreted by the processor. When the exe file is fed to the processor, it recognizes certain combinations of numbers as a command... And it performs the command it is expected to do.
The numbers I am talking about, is the machine code. The command the processor sees in it interpreted for human language is assembler instructions.
Disassemblers (like W32Dasm) play for processor, and read in all the numbers and try to make a corresponding interpretation of the numbers. This is called a disassembly.

Example:

Numbers: 	99 96 8F 29 00 01 76 CA 
Mach.Code: 
99
96
8F29
0001
76CA

Assembler Instructions :
99			cdq
		96			xchg	esi, eax
		8F29			pop dword ptr [ecx]
		0001			add [ecx], al

To make assembler listings faster understandable, we comment it. This is done by putting a ';' at the end of the line, and write whatever you want. 
Like this:

		99	cdq			 
		96	xchg	esi, eax	; exchange value in esi with eax
		8F29	pop dword ptr [ecx]	
		0001	add [ecx], al	; add value in al to address in ecx