RePEc is a uniquely organized initiative that brings with it an alphabet soup that confuses a lot of people, and we cannot blame them. Is a paper listed on IDEAS or RePEc? How is NEP different from RePEc? What is the difference between IDEAS and EconPapers? Etc.
For starters, RePEc (Research Papers in Economics, hence the capitalization) is a way to organize the data about publications of all sorts in Economics, and make all that available. Note that there is no central database, as every contributing publishers makes the data available on its own website following the rules set by RePEc. Beyond those rules, RePEc only maintains the list of pointers to where the publishers have put their RePEc archives.
Then, basically anybody can come and use that data. Some have decided to do that more formally and have their service listed in the repec.org domain. Examples would be EconPapers, IDEAS, and NEP. Others prefer not to or integrate the data in a larger scheme that spans more fields. Examples are Econlit, WorldCat, EBSCO, Google Scholar or ResearchGate. A third type of service uses part of RePEc data to enhance it and feed it back to RePEc. Examples for that are CitEc, EDIRC, and the RePEc Author Service. For a full list of the RePEc services that we know of, see the RePEc site.
Thus, RePEc is the basis for these services, to varying degrees, but they are independently run and RePEc has no say how they should be run. In fact, RePEc is not even a formal organization. Thus IDEAS is using RePEc data just as EconPapers is using RePEc data, but they are in no way directed by RePEc. And, the big difference between IDEAS and EconPapers is that they were initiated by different people. In the spirit of healthy competition, use the one you prefer.