According to the International Society of Genetic Genealogy, a haplogroup is a genetic population group of people who share a common ancestor on either their paternal (Y-DNA) or maternal (mitochondrial) lines.
Not every DNA testing companies show us our haplogroups. AncestryDNA, MyHeritage, and FamilyTreeDNA kits do not test for the specific SNPs required to determine haplogroup, while 23andMe and Living DNA do test segments of the mtDNA and Y-chromosome and display our haplogroups and, most importantly, those of our shared matches.
FTDNA also includes Y haplogroup and mtDNA haplogroup information, only if you buy specific tests related to those traits. Gedmatch displays haplogroup info that is self-reported.
I have found the haplogroup information displayed automatically at 23andMe to be invaluable in some of my research. In a future DNA Discussion meeting, I will give a show-n-tell of how this additional information helped unlock puzzles in our family trees.
Meanwhile, when searching the internet for haplogroup information, I found this page which has lots of comparative details about the various DNA testing companies. Eupedia is a great sight for exploring. And this page in particular should interest most everyone who has read this far! Do page through it.
My Y-DNA haplogroup is I1a-A13294 and my mitochondrial haplogroup is H1e1a.