STDIN is the name for one of the 3 standard file handles that a process inherits from its parent process. If you start a program from the command line without input redirection, STDIN is bound to the terminal, so that anything that's typed in at the terminal is delivered to the program via its STDIN handle.

But you can also start programs with STDIN bound to something else:

$ my-program < a-file
# or
$ some-program | some-other-program
[download]

etc.

In general when you open a socket you get/use a new filehandle especially for that socket, since even though it's possible to re-open STDIN in some other way later (through a socket or file or whatever) that would IMO just be confusing.

Though most unix systems have a fairly similar ways to deal with startup/shutdown scripts through various runlevel files etc, the differences in how to administrate those files easily are different enough that you should probably just look up your system's documentation.

If you want to become better informed about all these issues, I recommend Advanced Programming in the UNIX Environment (2nd edition).