Who is a "moser"? Do you know?

Scandal Shakes Up L.A. Orthodox Jewish Community
Forward (New York)
http://www.forward.com/articles/12542/
The scandal that has emerged since a Hasidic rebbe and others were charged late last month with defrauding the federal government of tax dollars has caused shock waves beyond Hasidic circles, with even Modern Orthodox rabbis addressing the issue in impassioned sermons... In traditional Jewish law, if a Jew reports another Jew to the government, he is deemed a moser, and in some interpretations, a moser's actions are punishable by death. The issue of mesira, or informing, has prompted a round of collective soul-searching in segments of Los Angeles's Jewish community. ◘