When a distinct Y-dna mutation is detected it is given a name usually beginning with a letter, an identifier for the lab that discovered it, and a number. There are other systems for naming such mutations, but I will use the ones most commonly used currently. For example, R-XX1234 simply means a mutation downstream from an older mutation named ‘R’, discovered in lab ‘XX’ where 1234 is simply a number that lab assigns to it.
Another site that maintains the recognized YDNA tree with an alternate nomenclature is ISOGG (International society of Genetic Genealogy)
The term ‘haplogroup’ (HG) simply refers to these mutations.
TMRCA refers to the ‘Time to Most Recent Common Ancestor’ which is a statistical estimate based on the number of mutations for a given Y chromosome lineage.
Bad Genealogy Rant: Thomas Shannon (1730-1794) and descendants. 