Look in your iTunes folder. You will see there is a media folder which contains the files iTunes plays. There are also various other files and folders which support what iTunes does. Most importantly the library.itl file which contains your list of music, playlists, ratings and basically what you see when you open iTunes.
If you move just media, you could open iTunes without the external attached and it would show you everything, but would obviously refuse to play anything without access to the files. If you turn on the NAS at this point it will still think the files are not there, though if you play one it would hopefully have another look and work. It will, however, think all the others are missing until you play each one, or restart iTunes with the NAS on.
There are other disadvantages to just keeping media on an external, mostly when it comes to moving the library in the future, or if you're in the habit of renaming your drives.
If you keep the whole iTunes folder on an external, if you start iTunes and the external is not on, iTunes will say it cannot find a library folder and default to the internal drive. If this happens you need to start iTunes with the option key as described in my earlier post. If you do keep the NAS on all the time, iTunes will always just start up with no problem. You could also use any computer with the NAs because the complete set of things needed for iTunes to work are on there.
Keeping it all in one place makes it a lot easier to move the collection in the future.
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html
iTunes 9 [and later]: Understanding iTunes Media Organization - http://support.apple.com/kb/ht3847
Image of iTunes folder structure Dec. 2011 - https://discussions.apple.com/thread/3547807?answerId=16923545022#16923545022
see also:
Image of folder structure and explanation of different iTunes versions (turingtest2 post) - https://discussions.apple.com/thread/2741246?answerId=13025536022#13025536022 and https://discussions.apple.com/thread/3700206?answerId=17457605022#17457605022
