It simply allows you to build a weapon, you still need to import the code into a proper save game editor like Rick's or WillowTree# before you can use it.
It can tell you if a weapon is legit or not (After the 1.30 Patch) though you can't always trust this. I've never had a legit weapon NOT work but some of the weapons that say they are not legit will actually work.
Seems to be something to do with the accessories, prefixes and titles. Also if you don't want the desktop version check out the gear calculator, that's the online version (No download needed)
Hope this helps.
*sadly i can't run that program keeps telling me i need to install adobe flash player which i have installed i am using 64 bit windows 7 btw dunno if that matters
[Edited by zyphoid, 10/10/2010 8:11:33 PM]