In population genetics, a haplogroup is a group of related haplotypes. However, in genetic genealogy, a haplogroup has come to mean a branch on either the maternal or the paternal tree of humanity. Specifically, for the paternal tree it is a branch on the Y-chromosome Tree defined by one or more binary polymorphisms. This shift in meaning is at least in part to to the way the term was used in the YCC 2002 paper by the Y Chromosome Consortium.
References & Suggested Reading: Wikipedia – Haplogroup