Guile's core language is Scheme, which is specified and described in the series of reports known as RnRS. RnRS is shorthand for the Revised^n Report on the Algorithmic Language Scheme. The current latest revision of RnRS is version 5 (see R5RS), and Guile 1.4 is fully compliant with the Scheme specification in this revision.
But Guile, like most Scheme implementations, also goes beyond R5RS in many ways, because R5RS does not give specifications (or even recommendations) regarding many issues that are important in practical programming. Some of the areas where Guile extends R5RS are: