Guile is the GNU project's implementation of Scheme, a Lisp dialect. Find out more at http://www.gnu.org/software/guile/guile.html. There's also some excellent information at http://www.red-bean.com/guile
The Guile docs are pretty good, but the Scheme standard (the wonderfully, obscurely named R5RS) can be found at: http://www-swiss.ai.mit.edu/~jaffer/r5rs_toc.html.
Guile is the scripting or "glue" language for the GNOME project. From the original announcement:
We plan to use GTK/Scheme bindings for coding small utilities and applications. When these bindings are more mature, it should be possible to write complete applications in Scheme.