What are the SQL standards
There are some general ISO/ANSI standards that should act as guidelines for all RDBMS that use the SQL language. The standards define features of the language, such as data types. The current standard is SQL 2011, which added a standard for temporal support.

Ideally, all RBDMS vendors that use SQL should develop language functionality according to the standards, making porting databases and user skills between the various implementations much easier. 

Unfortunately, this is not really the case. There are many features that RBMS vendors are slow to implement (e.g. SQL Server not having DATE until 2008, despite it being in the SQL:92 standards) and many things that the standards do not cover (e.g. indexing)

The full standards are not available freely online (I don't think), however you can read part one here SQL Part 1