Pyrrhotite is an iron monosulfide, unlike pyrite, which is an iron disulfide (FeS2). So, the chemical formula for pyrrhotite should be FeS. It isn't, however. The atomic structure of pyrrhotite often has holes - the mineral has an insufficient number of iron atoms. The formula for pyrrhotite ends up being Fe1-xS. In meteorites, iron monosulfide lacks the atomic-scale “holes” of pyrrhotite, and is called troilite (FeS).
Pyrrhotite is superficially like pyrite in appearance and chemistry, but they are definitely different minerals. Pyrrhotite has a metallic luster, a brownish-brassy or bronzish color, a black streak, no cleavage, and is magnetic. What’s particularly distinctive about pyrrhotite is that it is variably magnetic. The holes in the atomic structure gives pyrrhotite its magnetism. But, there's variation in the number of missing iron atoms from sample to sample. So, pyrrhotite ends up having variable magnetism. The more holes, the more magnetic the sample. The fewer holes, the less magnetic the sample.
Pyrrhotite has economic significance, as it often occurs with nickel-, copper-, and platinum-bearing minerals. A great example is the Sudbury Mining District in Ontario, Canada.
Pyrrhotite (all but the peacock iridescent areas - that's tarnished chalcopyrite) (field of view ~3.2 cm across) from Falconbridge, Ontario, Canada. This specimen is noticeably magnetic, but there's no magnetite component.