With MP3 and AAC most of the information is automaticcally stored both in the iTunes database library file and in the media files themselves as ID3 tags. Some things such as ratings, playcount are not stored in the files. If you want to include those you have to come up with tricks because there is no normal accommodation for that kind of data.
iTunes keeps a copy of artwork in its own artwork library, but you can optionally embed this artwork so it is both contained in the file and a copy being in the iTunes library. Of course this adds to the size of each file. The script indicated ensures that artwork that is only in the iTunes library also gets embedded in the file.
you mean that if i add the information directly from itunes and i use the consolidation of library, the id3 info will be add automatically into the files
If the songs are in iTunes and there is info, it is already in the ID3 tags.
Consolidating does not embed anything. It simply copies the file sint eh the iTunes media folder designated in iTunes prefs > Advanced.
When you add a file without artwork, iTunes will (usually) get the artwork. However, it is not embedded into the file. This script takes the artwork that iTunes has and embeds it (if it is not already there).
As Chris says this script will do it ( although I have had limited success as it crashes sometimes - maybe the size of my library ).
For individual albums, as stated above, select the album once iTunes has the artwork. In the Now selected window in the bottom left the artwork will appear. Right click in the window and copy and then Get Info on the multiple tracks and paste it into the artwork tab. This will then embed the artwork in the files themselves as opposed to just holding them in the iTiunes library files.