In Sims 2, we have one place.. A Downloads folder. That is NOT the case in Sims 1. There are other places to get this info, but this is MTS and we have everything here. So let's copy it here too.
The files are all labled IFF so name and know your files well. There are seperate folders for furniture, bodies, heads, and other things. Help me write the appropriate info. I'll edit this post later.

Walls are .wll and floors are .flr if I remember correctly

Skins, both clothing and heads go in c/program files/maxis/GameData/skins Both clothing and heads onsist of three files each (occasionally more if there are accessories): Body files/head files (a picture), files ending in "cmx" and files beginning with "xskin-".

Buyable skins go in a different folder, c/program files/maxis/expansion shared

The clothing images were bmp.
Houses were also iff
Roofs were bmp
animations were cfp....unless I'm thinking of something else
the x-skin files, started x-skin the extension was .skn
Pets were cmx, bmp and skn too

Custom npcs were cmx, bmp, skn and iff
The cmx bmp and skn went to GameData/Skins and the iff went to Game Data/objects/People (self made folder)

Phone plug-ins were iff
Careers were iff I think

I seem to recall sif files.....don't think you were meant to install these though...weren't these text type files?

FAR files....lol, sorry, I originally got these mixed up with sif files, and had to check my folders. I had thought sifs were the compressed large files, but it sounded wrong, so I checked it out.
Been bugging me since that last post what the compressed ones were called.
I just remembered it was FAR files, you could compress a bunch of iffs into one FAR file, don't think many people used them though, and can't remember if there was a reason why people stuck with loose iffs instead.
It could just have been that it was easier for people to remove one part of an object set you didn't want and keep the rest rather than having an item you didn't want clogging up your game or removing the whole set to get rid of the one item.
Probably was this, since I think the files that were most often compressed into FARs were the ones that required all the iff files to run the mod anyway.

I used a lot of fars - it saved a lot of space. I used something like Far Out to make them. Basically, they're a bit like zips/package files.