xml in SAPI

i installed sapi5.1 and succeded in running both reognizer and tts,
but there's a point i didn't understand:
what does xml files do in recognizing?
With the sample program of recognizing which came with sapi there are a lot of xml files that can be used as Grammar files, but altough i looked at the xml codes (-i don't know xml!-), i didn't understand how these files can be use as Grammar
if it'll be usefull for you i can send the working codes that i wrote

Thanks to everyone who says something:))