Haplogroups are simply mitochondrial or Y chromosome DNA lineages that trace back to a single SNP which arose in one person at a specific place and time, and was subsequently passed on to all of that individual's descendants. By assembling hundreds of such SNPs, geneticists can create a tree of haplogroups that ultimately traces back to the common ancestor of everyone's mitochondrial or Y chromosome DNA.
Our Ancestry Tools feature, Haplogroup Tree Mutation Mapper, allows you to see which SNPs are used by 23andMe to define each of the hundreds of maternal and paternal haplogroups. For each haplogroup, the SNPs that define the most recent branch are listed first, followed by SNPs that define older branches, all the way back to the SNP that defines the root. The Haplogroup Tree Mutation Mapper is especially useful for advanced users who are interested in understanding exactly which SNPs are used by 23andMe to define each haplogroup.