A haplogroup is a group of similar and ancestrally related haplotypes that all have the same single nucleotide polymorphism (SNP) mutation. It is a genetic marker for a group of organisms with a common ancestor.
In human genetics, the haplogroups usually studied are Y-chromosome (Y-DNA) haplogroups and mitochondrial DNA (mtDNA) haplogroups. Both can be used to define genetic populations. Y-DNA is passed only from father to son, while mtDNA is passed only from mother to children. Neither recombines, and thus Y-DNA and mtDNA change only by chance mutations with no intermixture between parents' genetic material.
Haplogroups are used in some forms of genetic ancestry research such as research determining the most common haplotype(s) in different populations and argued genetic ancestry relationships between the populations based on this. However, haplogroups are not races or some kind of racial "essences" that define races. Y-DNA and mtDNA haplogroups represent only a very small part of the human genome.