REM

In a batch file
REM signifies a comment or REMARK
adding :: at the start of a line has a similar effect

For example:

@ECHO OFF
::
:: First comment
::
REM Second comment
REM
::
Although you can use rem without a comment to add vertical spacing to a batch file, you can also use blank lines. The blank lines are ignored when processing the batch program.

The double-colon is not documented as a comment command, it is a special case of a CALL label that acts like a comment. The pro's and cons of each method are listed below.

Bugs
There are problems using a :: comment within an IF or FOR code bracket
e.g.
@echo off
FOR /L %%i IN (1,1,10) Do (
Echo before comment
:: Some comment
Echo after comment
)

The above will return the error :: was unexpected at this time.

In the script below the DIR command will set an %errorlevel%=2 if the file is not found, but the REM command is then executed successfully and resets %errorlevel%=0
If you use :: for the comment the errorlevel stays at 2.

DIR nonexistentfile.txt
REM some comment
ECHO %errorlevel%

The problem above was fixed* in Win XP and later service packs of NT 4.

Finally in Windows 2000 and XP a comment like
::%~
or
REM %~ will be interpreted giving the error:
The following usage of the path operator in batch-parameter substitution is invalid: %~

The bottom line on this is that you must test your comments to be sure they will be ignored as you expect.

Registry Comments

Within a registry file comments can be preceded by "; "
e.g.
;
; Turn the NUMLOCK on at login
;
[HKEY_CURRENT_USER\Control Panel\Keyboard]
"InitialKeyboardIndicators"="2"

FTP Comments

There is no valid comment character for FTP but you can cheat by escaping to the shell and running REM
e.g.

C:\WORK>type ftpscript
!REM This is a remark
bye

C:\WORK>ftp -s:ftpscript
ftp> !REM This is a remark
ftp> bye

C:\WORK>

* The errorlevels set by DIR are different under Windows NT 4 and XP

Equivalent Linux BASH commands:

### - Comment / Remark