nexus is the software producer. Offical botting sites are just resellers such as perfectious and rsbots. They resell nexus's software and adapt it slight so it is there own.
edit : both parties profit from this. Not sure how they work it it might be 50/50 but I know not all the proceeds go to either nexus or the botting party. They split the money
NeXus is the client, made by IMPSoft.
Scripts are written by script developers, such as Perfecticus, Phil, and Slade.
Scripts are resold by websites such as RSBots.net, and the developers get a cut of the profits.