The major difference between the Y-DNA test and the autosomal DNA test (like the Ancestry test, that most people start with) is that the Y-DNA test gives information about your deep paternal ancestry going back tens to hundreds of thousands of years back. As females do not have a Y-chromosome, and exact copy of the Y-chromosome is passed down from father to son, unlike any of the other 22 chromosomes which are a mixture of the mothers and fathers DNA, and thus gets diluted every generation. On average, every 140 years or so a single mutation occurs in the Y-chromosome, and this mutation is then inherited by all future sons. Using this information, we can trace DNA back to a single common shared ancestor from Africa about 240,000 years ago. This ancestor’s (known as Y-Adam) sons’ migrated in small groups out of Africa, and their sons picked up new mutations on the way leading to many distinct DNA signatures. These are known as haplogroups and can be identified by this Y-DNA test. Depending on which haplogroup you are, you can trace your paternal ancestors journey out of Africa to the present day.
The results of my own Big Y-700 DNA test which show the route my paternal ancestors took to arrive in England between 1300BC and 1600 AD. In addition to this information, the Big-Y test also identifies any unique mutations (SNP’s) I have which are not shared by others. In the block chart below, I have only 1 match (my son) who is in exactly the same block (subclade) as myself, but I also have some close paternal relatives with the surname, Hadley, Purnell & Daniel. We would have shared a paternal ancestor sometime before 1600, when our particular subclade originated (from a single point mutation in the Y-chromosome). As more people (and more closer relatives get tested), this information will get more granular.