What is PMAed, STDed, etc.?

In researching after-market instrument lights (see:

http://groups.google.com/groups?hl=e....tagonline.com

), I've run up against some terms that I think I should understand better.
These are "STCed" and "PMAed". I understand that they're somehow related to
having these lights (or whatever is under discussion) approved for installation,
use, or some such in an aircraft. But I think I need to understand this better.

STC stands for "Supplemental Type Certificate". This "certifies" a
modification of an aircraft from its original design.

PMA stands for "Parts Manufacturer Approval". This is an approval of a
replacement part.

I don't quite have the "big picture". For example, the Nulite web site
indicates that all their products carry a PMA certification. Thus, again
according to the site, "any certified person" can make the appropriate log
book entry and the installation is "legal". That's right? Is "any
certified person" an I&A?

But I'd be using the Nulites not as a replacement for anything, but as
a new lighting solution. Does that make a difference? That is, does this
mean that a PMA is insufficient, and that an STC is required?

If not...when is an STC required instead of a PMA?

Thanks...

Andrew