How many kinds of photo paper are there?

[The Hat]

I think that photos on display indoors exposed to the effects of CFL UV lights has a huge problem with fading, never mind the sun..

 

[martin0reg]

It is the coating (the top layer) which is either porous or swellable.
I found these links from german sites, hoping the pictures are self-explanatory.

There are the two different types how the droplets are absorbed by the coating.
http://www.druckerchannel.de/artikel.php?ID=1630&t=know_how_fotopapierarten

And there are three types of making the coating
http://fineartprinter.de/de,03e9,03e9,103;doc.html