please i want to know ingeneral the difference between Client and Server Side and specially Difference between "SFTP Client Side" and "SFTP Server Side" and where i should be install SFTP client and SFTP server i i want to copy file by sftp command from server X to Server Y ?????
SFTP is FTP over SSH so if both servers are servers then both most likely already have SSH (if they are unix servers)
You then need an SFTP compatible client.
Since they are both servers It does not really matter which one has the client and which one has the server.
Typically one would use a client on a client machine, be it windows, Mac or linux to connect to the SFTP server on the server but in your case both are servers? then it really does not matter. one does the connecting while the other does the listening.
Perhaps you can provide a bit more info?
If they're both Unix or Linux servers then most probably both of them already feature an SSH server (and therefore, if properly configured, they're capable of acting as a SFTP server).
The rest of the answer lies in the question itself: instead of calling them "server X" and "server Y", let's call them for a moment "computer X" and "computer Y", in order to make sure the word server doesn't confuse us. Now, according to your question computer X has to send files to computer Y: therefore the most common scenario would suggest that:
In all fairness it doesn't really matter, as nothing prevents you from running the client on Y and the server on X, and pulling the files from X (downloading them) instead of pushing them to Y (uploading them). But in the scenario that you describes, it seems just more obvious to run the SFTP Client on Server X, and the SFTP Server on Server Y.