Are there any differences between Master's admissions and Ph.D admissions in the US? I notice that the majority of admission questions on this site regarding admissions specifically refer to Ph.D admissions. That being the case, are Master's applicants evaluated any differently in regards to focus on research, grades, letters of recommendation, etc.?  For instance, research potential is a big determining factor for Ph.D admissions I know, but the Ph.D is a research oriented degree. Is there the same focus on research potential for Master's students? 

This is specifically focused on STEM fields, particularly the "E."
The difference is huge, at least in my field, computer science.  Master's programs are typically money-makers, with students paying full tuition.  Some programs are purely coursework; others require Master's theses or projects.

In contrast, PhD programs accept many fewer students, none of whom pay tuition, instead receiving stipends paid for by fellowships, teaching assistantships, and research assistantships.

Because of the difference in the size of these programs and who pays whom, PhD programs are far more selective.