District judges are appointed for life by Congress, Magistrate judges
are appointed for 8 years on the basis of merit. Magistrate judges
cannot try felonies. They try cases in US District courts.
see url: https://www.google.co.uk/#safe=off&q=what+is+a+u.s.+magistrate+judge
and/or url: https://ballotpedia.org/Federal_magistrate_judge
for a paper on the system and powers of a Magistrate Judge.