A Haplogroup is a branch of the human family tree. All men in the same Y-DNA haplogroup share the same SNP or SNPs (unique marker/s in the Y-chromosome) which they have inherited from their common ancestor. The haplogroup is like a name for that common ancestor person. The haplogroup allows to research the modern distribution and the construction of a migration hypothesis of the descendants of the haplogroup founder. Source: https://isogg.org/wiki/Y-DNA_project_help