The Y Chromosome is the single largest piece of DNA inherited as a block, and always coming from the father, it is a very useful tool in uncovering male ancestors. It remains largely unchanged as it passes from father to son, as it does not have a matching chromosome to recombine with.
The Y Chromosome is very interesting, because such a big block is inherited together, there is about one new genetic variant per generation or two. This means that complete Y Chromosome sequences should, in theory, allow us to reconstruct a family tree of all men on earth, accurate to a couple of generations. In reality, it is not simple to read all the letters of the Y chromosome; however a very refined tree can be built. As we improve resolution, the potential for improved interpretation increases too.