Why do package names often begin with "com"

Possible Duplicate:
Java packages com and org

I am a java developer. Nowadays I am learning struts and when reading a tutorial a curiosity intruded in my mind regarding

package com.something.something;

I know it is a very simple package declaration but what about

package **com**.something.something;

This package name fragment often comes in many commercial distributions. Now I want to know what does it mean? Please clarify it.

Thanks and sorry if I couldn't clarify it...