WARNING: The Transfer language is still experimental. Its syntax, type system and semantics may change without notice. I will try to help you with any problems this might cause, but I will not refrain from changing the language solely for reasons of backwards compatibility.
The GF Transfer language is a programming language which can be used to write functions which work on abstract syntax terms.
The Transfer tutorial shows an example of how to write and use a simple transfer function for a GF grammar.
The Transfer reference aims to cover all constructs in the Transfer language.