What is *.xs file?

[Sam Xie]

Hi! There,
    I am learning to write a perl module.  I saw someone's AFS.pm module, 
in which, he wrote a AFS.xs file in C. I don'e know what .xs extension
means and how to write it?  If someone knows it, it will be gratefull to 
help me to understand it!
Thanks!
Sam