Wrote a batch file just now to do some DB setup for a Whidbey application that needs some ASP.NET features like Membership and Personalization.  So it has to call aspnet_regsql.exe a few times.  I used to write a lot of UNIX shell scripts, so I forgot what the comment character was for .bat files (hint, it wasn't '#').  In fact, it's not a character at all, but the lovely 'REM'.  Found this handy reference which includes the allowable commands:

• @
• Do not echo this line
• ECHO
• Send messages to user
• Turn on/off output to user
• REM
• Comment out a line (short for remark)
• PAUSE
• prints "Press any key to continue..." and waits
• GOTO
• branches to a label
• IF
• can be used to check if a file exists or if a command returned an error
• SHIFT
• shifts command line params (useful if you have a lot of them I guess)
• CALL
• run another batch file from this one; allows reuse
• FOR
• allows looping
• CHOICE
• Prompts the user with a menu of choices and accepts one

Obviously I'm not offering a ton of syntax here -- go to the reference for that.