Hackers are the people who break into computers. Hackers get in to a computer using an exploit to get in via a program's bug. An exploit is a script which uses the bug to get in the pc. The program/exploit he uses (of course) depends upon the vulnerability the scanner has scanned for. When in, the hacker runs installs a rootkit. This rootkit (usually a modified version of Serv-U) is the server where other people can download from. Most likely he will also install remote administrator software (usually Radmin), so he can re-enter the computer easily. Once the server is installed and working he'll post the admin login data to the FTP server on his FXP board. Depending on the speed of the compromised computer's (aka pubstro or stro) internet connection and the hard drive space, it will be used either by a filler or a scanner. The hackers from these fxp-boards are rather good, and are capable of hacking 100mbit's.