exporting library

You can export a playlist and import it to a different computer so that you can have the same playlist in both places. You can also export a playlist to save a copy of it.

Exported playlists contain a list of the items in the playlist; the actual songs or videos aren’t included.

You can save all the information about the songs in a playlist or your library as a plain text file so that you can import it into a database or use it in another application. You can also copy just one song’s information into another application.

Here are ways to save copies of playlists or use playlists on another computer:

• To save a copy of a single playlist or use it in iTunes on another computer, select the playlist (below Playlists), choose File > Library > Export Playlist, and choose XML from the Format pop-up menu.
• To import an iTunes playlist, choose File > Library > Import Playlist. The imported playlist includes only songs and videos already in your iTunes library. Unavailable items are removed from the list.
• To save a copy of all your playlists, choose File > Library > Export Library. The exported information is saved in XML format.

From http://support.apple.com/kb/HT4527 > Home Sharing

"Transferring playlists

If you want not only your content but also your playlists to exist on your new computer, you will want to transfer a copy of all your playlists from your old computer. To do this, choose File > Library > Export Library on your old computer. Save the XML file that is created to your desktop."

No, it does not save a a copy af any song.
It save the playlist only (a list of songs).

Export playlist just makes a copy of a playlist you have selected. Export library makes a copy of everything in the library. These are text listings, not the actual media files.