At FamilyTreeDNA, three levels of Y-chromosome STR testing are offered to project members:  37-, 67-, and 111-marker tests.  Every level of testing tells you something, but I have found that 67-markers is usually needed to have confidence a match isn't coincidental, so I have standardized all my projects on 67 markers.  Those of you who turn out to be any form of Haplogroup R1b will likely need to test 111-markers to have full confidence their geneatic matches are not coincidences.  

STR testing measures relationship in a time frame of hundreds of years, making it sufficiently close to be genealogically useful.  And "useful" is an understatement.  STR testing is a powerful tool for determing whether people do or do not share a recent common ancestor.

There is another form of Y-chromosome DNA testing called SNP testing — the acronym stands for "Single Nucleotide Polymorphism" and is pronounced as a single word, "snip."  Results of SNP testing are expressed as + or - (positive or negative, derived or ancestral)) to indicate the presence or absence of a particular mutation.  Each suspected mutation requires a separate test, though FTDNA offers these tests in "bundles."  Results of SNP testing determine a person's haplogroup, and deep testing (meaning exhaustively testing all known SNPs for the haplogroup) can identify subgroups, called "subclades."

The haplogroup (sometimes abbreviated, "Hg") is a measure of deep ancestry.  The MRCA (Most Recent Common Ancestor) may be thousands of years in the past, which places the individual's ancestry in a paleoanthropological time frame.  Haplogroup determination is of practical use for the project because it broadly defines and separates the most distinct family groups; that is, it separates families with no prospect whatsoever of a connection in "genealogical time," making it a useful way to divide up the project.  For the individual, knowing one's haplogroup subclade greatly enhances one's appreciation of history because the literature on the genetic history of humanity, correlating haplogroup subclades with historic and pre-historic events, is growing rapidly.

Speaking personally, compiling my genealogy greatly enhanced my appreciation of U.S. and European history, while discovering my parents' haplogroups has enhanced my appreciation of the global history of humanity.  I wish I had known these things about myself when I was growing up, but at least the future children in the family will know.

As each individual has just one evolutionary path on their patrilineal line, so their SNP-based haplogroup should correlate with their STR-based haplotype, and it does.  If the haplotype is reasonably common (or similar to one that is), it can be used to deduce the probable Y-DNA haplogroup, without the added expense of SNP testnig.  If the haplotype does not unequivocally indicate the haplogroup, FamilyTreeDNA will do a "backbone" (basic) SNP test without charge to securely determine the basic haplogroup.  Deep SNP testing would still be needed to determine the haplogroup subclade, and if your haplotype is rare, I recommend deep SNP testing simply as an aid to researchers because their research on your haplogroup ultimately tells you more about yourself.  I especially recommend deep SNP testing if you are Haplogroup R1b because this is the most common haplogroup in Europe and has many subclades.

Haplogroup subclade determination via deep SNP testing is (to me) a fun thing to know about oneself — and it does help me to organize project members — but it is not a requisite for participation in this project, which is based on STR testing in a genealogical time frame.