/sigh.
RSLinx, in it's various persona's, is a communications program, designed to allow various pieces of software to communicate on a native level with AB/Rockwell hardware, primarily PLC Processors, and associated IO.
RSNetworx is a programming and diagnostics tool, used to configure and troubleshoot some of the higher end (primarily Devicenet and ControlNet) networkable devices. RSNetworx for Ethernet is almost completely unnecessary, as Rockwell Ethernet devices cannot and need not be configured through networx.
Then see OZEE's response above.