That's a Windows command.
For information on Windows commands, refer to the excellent Command-line reference help file (ntcmds.chm) included with Windows. This file is normally found in the %windir%\Help folder, which is C:\WINDOWS\Help on most systems. You can click on the ntcmds.chm file directly in Windows Explorer, or you can run it from the Start;Run dialog or any command prompt by typing: hh ntcmds.chm
Once the Command-line reference is running, click the Command-line reference A-Z to see an alphabetical reference to all Windows commands.
There you'll find a detailed reference to the command you are looking for, including examples.
Note that the NET SEND command requires the Messenger service to be running, and that service is disabled by default in Windows XP SP2 and later, and is not supported at all in Vista:
You can enable the service in XP, however that will open your system up to messenger spam (which is why it was disabled by default in XP SP2 and later):