The EndNote library
An EndNote library is made up of a file with the extension *.enl plus a corresponding *.DATA folder. Both units make up a complete library, and they both need to have the exact same name and need to be located at the same location, see picture below.
Backup of the library
The simplest way to send (or make a backup) of your library is via the "Compressed Library"-function in EndNote.
You find it in EndNote X9 under File>Compressed Library (.enlx)...