The type of Y-chromosome markers measured by genealogical genetic testing labs are known as STRs. Genealogists are interested in finding connections between families on a time scale of centuries, and the mutation rate of STRs is such that they are a good choice for that kind of work. Population geneticists are interested in tracking the movements of groups of humans over time scales of 1000's or 10,000's of years. Therefore their studies usually involve a different type of Y-chromosome marker known as SNPs (along with insertions and deletions) which have a much slower mutation rate than STRs. Haplogroups are defined by patterns seen in the alleles of these slowly mutating SNP markers. Identification of your Y-chromosome haplogroup can provide an interesting glimpse into the deep ancestry of your paternal line.
A SNP test would be the only way of identifying one's haplogroup for certain. However some conclusions can be drawn about haplogroup classification by looking just at the STR marker value patterns. Whit Athey has put together a Haplogroup Predictor that uses these STR patterns to give estimates of Y haplogroup.
A quick rule of thumb for estimating haplogroups is available here.
Searches for new SNP's have only been conducted in about 1% of the length of the Y chromsome - so much remains to be discovered. Most of the haplogroups that have been identified to date are more than 10,000 years old - which means that enough migration has occurred since then that most haplogroups are seen to one degree or another all over the European continent. This limits their utility in trying to determine a place of origin for your line. As more research is done, younger haplogroups will be discovered that have a more limited geographic range, and we may soon be able to learn more specific geographic information from haplogroups.
The links below provide a summation of recent research on the more common Y haplogroups.
More details:
Allele frequency graphs by haplogroup
Haplogroups in the YSTR.org Database
Haplogroups in Spain and NW Africa
Haplogroups in California and Hawaii
20 Marker haplotypes for worldwide YCC samples
