If you're one of the many who's done deep ancestry testing (say, through the Genographic Project), but don't understand what that letter you've been assigned means, take a look at Georgia Bopp's Haplogroup Chart that boils it down to the very basics -- that is, the locales for major Y-DNA and mtDNA haplogroups. And if that leaves you wanting more, scroll down and click through to some of the map and migration links offered. Handy resource, Georgia!