CDA vs. NDA vs. CDEV

[gachenb@eis.calstate.edu (Greg R. Achenbach)]

Well, I finally got some extra memory for my GS and am starting to 
explore the world of GS software.  One question I have (actually I guess 
it is several questions) is:

What is the difference between an NDA, a CDA, and a CDEV?  
What or the pro's and con's of each?  
How do I use them?  
Once I've downloaded one, where do I put it on my disk?

Thanks in advance.
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Greg Achenbach             gachenb@eis.calstate.edu (soon to be ctp.org)
Los Angeles County Outdoor Science School

"Society is like a stew.  If you don't keep it stirred up, you get a lot of
scum on top."  Edward Abbey