Guile is a library designed to help programmers create flexible applications. Using Guile in an application allows programmers to write plug-ins, or modules (there are many names, but the concept is essentially the same) and users to use them to have an application fit their needs.
There is a long list of proven applications that employ extension languages. Successful and long-lived examples in the free software world are Emacs, the GIMP and GnuCash.
Very popular examples of extending server applications are the Apache projects Perl and PHP modules.
Extension languages allow users, programmers, and third-party developers to add features to a program without having to re-write the program as a whole, and it allows people extending a program to co-operate with each other, without having to expend any extra effort.
Guile also has an object system called GOOPS
The Guile homepage can be found here: http://www.gnu.org/software/guile/guile.html
If you have old code that uses Guile, ttn maintains a backward-compatible Guile 1.4.x and related programs.