Below are a couple brief answers given by FamilyTreeDNA to the question "What is a haplogroup?"
"One way to think about haplogroups is as major branches on the family tree of Homo Sapiens. These haplogroup branches characterize the early migrations of population groups. As a result, haplogroups are usually associated with a geographic region. If haplogroups are the branches of the tree then the haplotypes represent the leaves of the tree. All of the haplotypes that belong to a particular haplogroup are leaves on the same branch. Both mtDNA and Y-DNA tests provide haplogroup information, but remember that the haplogroups nomenclature are different for each."
"The haplogroups are the major branches on the Y chromosome tree, defined single nucleotide polymorphisms (SNPs), which have accumulated along different lineages as Y chromosomes are passed from father to son over many generations. All haplogroups ultimately descend from a single Y chromosome carried by a male that lived in the distant past. The topology of the Y chromosome tree can be reconstructed by typing mutations in different human populations – as more SNPs are discovered (e.g., M254), the structure of the tree changes. Originally, the Y Chromosome Consortium (YCC) arbitrarily defined 18 haplogroups (A-R), which represent the major divisions of human diversity based on Y chromosome SNPs. Currently there are 20 haplogroups (A-T). In turn, each of these major haplogroups has numbered subgroups, or subclades, that are named with alternating letters and numbers."
Y-chromosome DNA Haplogroups of Man
Below is a table of yDNA Haplogroups, consisting of many sub-groups descending from what are know as "super-haplogroups", such as F, K, and P.
| Y Chromosomal Adam (Noah) | ||||||||||||||
| A | B | C | * | F | ||||||||||
| D | E | G | H | * | K | |||||||||
| I | J | L | M | * | P | |||||||||
| N | O | R | Q | |||||||||||
For more details about what Haplogroups are, click here to be directed to Wikipedia's portal on the subject.