SQL, or Structured Query Language, is a widely accepted protocol used for data access. It is an ANSI standard with SQL-92 being the most recent specification. A summary of the language appears in Appendix B.
The language is defined with various levels, listed in the appendix as minimum, core and extended. All SQL servers support at least the minimum level, some may also support core or extended levels, or extensions of their own. ODBC DBMS drivers are distributed with Help files that list the functionality of the driver, plus other useful information - when you install the drivers you should also print out the Help files for reference.