Not specific to EPJ, but yes, conference paper review is different from most journal review due entirely to time constraints. And the proceedings published by EPJ have their own review processes, but are likely to be more time constrained than journal review.
Journal papers can take a long time to review, by several reviewers and with several iterations of (hopefully) improvement. This isn't possible for conference proceedings due to the short time between the end of the call for papers and the conference itself.
So, for conference papers, one round of review with an accept/reject decision by the program committee is pretty typical.
In some fields (CS) conference proceedings are the main vehicle of new result dissemination. In others, conferences are much more informal.
But both sorts of papers are refereed by knowledgeable people in my experience.
If you want to split these for a CV as might be appropriate in Physics, consider "Journal Publications" and "Conference Proceedings" as the headings rather than making value judgements on the rigor of the two systems. Those in the field will likely know of the different review processes.
Note that there is a third option for conference paper review, called shepherding, used in the patterns community. In this system, a "shepherd" is assigned to a paper who works closely with the authors to improve the paper. The work can have several iterations over a short period, and the shepherd and authors are (obviously) known to each other. It has some of the advantages of journal review, though isn't blind, providing several feedback iterations. Shepherds are typically knowledgeable about the subject of the paper and the process itself.
Also note that special issue journal review also may have tight time constraints, similar to conference review.