newbie question about EXTENT

[Douglass Davis]

Ok, I must admit, I am a PHP/Web programmer not a GIS expert...

But, looking at the mapfile, what are the units used for EXTENT??  I 
mean, I know its x and y values, but what do they represent?

For example.  What does this mean exactly, and how does it relate to the 
shapefile, and the image output of the mapserver:

    EXTENT -2200000 -712631 3072800 3840000


The docs only say they are x and y values.