RESX is one of the formats for 'Resource' files. Generally, they are used for localization.
RESOURCES are binary files embedded for CLR to use at later stage.
Here: MSDN: Resources in .Resx File Format
Few more details here: Introduction to The Resources .resx and Resources Files: Part I
It too says, RESX is a kind of structured XML format file, such as the XSD files those used to stock information about datasets elements and structures. It is, normally, used for structuring and organizing data in a given order.
RESOURCES is a kind of file as a common language runtime binary file that one can embed within a runtime environment. In order to be used by the application core later.