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. Guile complies fully with R5RS (see Introduction in R5RS), and implements some aspects of R6RS.
Guile also has many extensions that go beyond these reports. Some of the areas where Guile extends R5RS are: