Guile implements Scheme as described in the Revised^5 Report on the Algorithmic Language Scheme (usually known as R5RS), providing clean and general data and control structures. Guile goes beyond the rather austere language presented in R5RS, extending it with a module system, full access to POSIX system calls, networking support, multiple threads, dynamic linking, a foreign function call interface, powerful string processing, and many other features needed for programming in the real world.
The Scheme community has recently agreed and published R6RS, the latest installment in the RnRS series. R6RS significantly expands the core Scheme language, and standardises many non-core functions that implementations—including Guile—have previously done in different ways. Guile has been updated to incorporate some of the features of R6RS, and to adjust some existing features to conform to the R6RS specification, but it is by no means a complete R6RS implementation. See R6RS Support.
Between R5RS and R6RS, the SRFI process (http://srfi.schemers.org/) standardised interfaces for many practical needs, such as multithreaded programming and multidimensional arrays. Guile supports many SRFIs, as documented in detail in SRFI Support.
In summary, so far as relationship to the Scheme standards is concerned, Guile is an R5RS implementation with many extensions, some of which conform to SRFIs or to the relevant parts of R6RS.