From ISOGG Wiki
A Y-STR is a short tandem repeat (STR) on the Y chromosome. Y-STRs are usually designated by DYS numbers. The standard Y-chromosome (Y-DNA) test used for genealogical purposes looks at differing numbers of Y-STRs, collloquially known as markers. The combination of Y-STRs tested is known as a haplotype.
Y-STR allele nomenclatures
DNA testing companies or labs in certain cases use different nomenclatures to designate the same Y-STR allele. Thus, a conversion must be applied in these cases to accurately compare Y-STR results obtained from different companies. The most common nomenclature is based on guidance provided by NIST for Y-STR markers historically reported differently by various companies. The NIST standard is the proposal of ISOGG for genetic genealogy companies. 
- from FTDNA to NIST: In FTDNA Projects the Haplotypes (STR Values) are formatted in the FTDNA lab standard (v1 until 2008, v2 since 2009?). For conversion from FTDNA v2 to NIST (ISOGG) standard used in scientific studies the following conversion apply:
DYS441 +1, DYS442 +5, YGATAA10 +2, GATA H4.1 +1.
- from NIST to FTDNA: DYS441 -1, DYS442 -5, YGATAA10 -2, GATA H4.1 -1. Sometimes DYS389II-I is listed; both values need to be summed.
- from ISFG to FTDNA: same as for NIST except for GATA H4.1 -10 (-9-1)
See also the article by Diana Gale Matthiesen on Converting Y-DNA STR Test Results between SMGF, AncestryDNA, and FamilyTreeDNA
Y-DNA STR frequencies
- Y-STR allele frequencies by haplogroup, a resource compiled by the late Leo Little
- SMGF marker details - a chart showing the frequency of Y-STR alleles in the Sorenson Molecular Genealogy Foundation database (from the SMGF website preserved in the Wayback Machine
- Histograms showing the frequency of Y-STR alleles in R1b Compiled by Charles Kerchner and Tom Cloud.
- R1b-L21 allele frequency A chart compiled by Mike Walsh showing the allele frequencies in L21 based on results of over 6000 members of the L21 project.
- National Institute of Standards and Technology (NIST) standard reference database
- National Institute of Standards and Technology (NIST) (US) — Y STR positions along Y Chromosome
- National Institute of Standards and Technology (NIST) — Summary list of Y-chromosome STR loci and available fact sheets
- National Institute of Standards and Technology (NIST) — STR Fact Sheets
- National Institute of Standards and Technology STR training materials
- Genebase Y-STR marker types tutorial
- List of DYS markers
- Marker standards SMGF table preserved in the Wayback Machine
- Y-DNA Testing Company STR Marker Comparison Chart
- Matching multicopy Y-STR markers in closely related individuals A presentation by Thomas Krahn of Family Tree DNA]
- An Interview with John Butler and the Human Identity Group of U.S. NIST Journal of Genetic Genealogy, Fall 2008, Volume 4, Number 2.
- "Marker Standards". Sorenson Molecular Genealogy Foundation (Wayback Machine). https://web.archive.org/web/20150418134725/http://www.smgf.org/ychromosome/marker_standards.jspx. Retrieved 15 May 2015.
- Matthiesen, Diana Gale (19 July 2011). "Converting Y-DNA STR Test Results between SMGF, AncestryDNA, and FamilyTreeDNA". http://dgmweb.net/DNA/General/ADNA_FTDNA.html. Retrieved 11 August 2012.