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 GNU Emacs and The GIMP.

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 is an interpreter for the Scheme programming language, packaged as a library which can be incorporated into your programs. Your users have full access to the interpreter, so Guile itself can be extended, based on the needs of the user. The result will be a scripting language tailored to your application.

Using Guile with your program makes it more usable. Users don't need to learn the plumbing of your application to customize it; they just need to understand Guile, and the access you've provided. They can easily trade and share features by downloading and creating scripts, instead of trading complex patches and recompiling their applications. They don't need to coordinate with you, or anyone else.