XDCC (Xabi DCC or eXtended DCC) was initially a script written in for ircII by Xabi. This script extends the ircII DCC command.
In today's world XDCC refers to IRC bots running file sharing programs. XDCC bots serve one or more usually large files for download using the DCC protocol. It is widely understood to be a protocol for which illegal content (usually illegal MP3 files or other forms of warez such as pirated software) can be listed and, subsequently, downloaded it is also used in the legal sense. Many XDCC servers are hacked.
Frequently, FXP groups utilize connections with large upstream bandwidth (e.g. Colleges and Universities) in order to mass distribute warez releases (usually ISO images compressed using the RAR compression algorithm, with other files (NFO and SFV) packaged together as a tar file).