FXP is the common name for transferring data between two ftp servers (let's call them A and B) without the data going through the client (C). This is possible because because ftp uses two ports, one for control and one for data. The client asks server B, the destination server, to open a connection for receiving data and records the IP and port the server gives out. The client then asks server A, the source server, to send data to that IP.
In a world where everyone had the same amount of available bandwidth and diskspace, FXP would be a bit pointless. The way the internet is structured today, however, there is a relatively small amount of people who have high-speed links which can be used for extralegal traffic, and a relatively large amount of people who have medium- to slow-speed links. Thus is born the concept of "FXPing", or sending (mostly copyrighted and thus extralegal) data between different servers, and so gaining credits which can be used to fxp more data.
In the beginning there was data, and people shuffling it around
- William Gibson