I would really appreciate if someone could point me in the direction of a resource to teach me how to do this. I open the package files with S3PE to take a peak and it's all gibberish, cept for the snaps cause those are obviously the pics. I'd like to be able to look at the files in S3PE and know what they are and what they do. I've not been able to find any kind of guide or how to on this despite much googling so I would much appreciate help! Just to clarify - I'm not wanting to bundle packages, I know how to do that. I just want to know what the files mean.

I'm pretty sure most of the stuff is written by a package editor, something like TSRW and not really anything you edit manually unless you have previous experience. You can edit stuff like the ITUN and CASP entries as the data in there is fairly self-explanatory and using an image editor with a dds file plugin lets you edit and create new textures.

A few links to get you started:
http://modthesims.info/d/browse.php?f=139&gs=2 (There's honestly so many things you can do, that for a lot of things there's specialized utilities, which you can find here)
http://simswiki.info/wiki.php?title=Sims_3:Modding
http://modthesims.info/wiki.php?tit..._Tuning_Modding
http://sourceforge.net/projects/sims3tools/
http://www.thesimsresource.com/workshop/

EDIT: If all you really want to know is the different resource types, then check this list for details: http://simswiki.info/wiki.php?title...PackedFileTypes (Realized after I got all those links that this might be all you're really wanting, doh )

You'll get much more useful info if you say what kinds of files you're opening, and what you want to do with them. After all - there are probably about 100 different resource types in sims 3 files, and they all work differently; I doubt you want to learn how to interpret and modify every single one .

Thanks kikaimegami for all the links. Some of them are interesting and good information but I was not specific enough. Sorry. W

Nysha, what I am specifically trying to do is open a package file or a sim file with S3PE and be able to know what all those files are on the inside. I'm not interested in learning to mod or edit the files, I just want to know what they are. For instance, I opened a package file and saw these tags --- BBLN, BGEO, BOND, BONE, GEOM...and so on. I'd like to know what those abbreviations mean, if it's a good or a bad thing, and see what actually is going into my game before I put the package in. From your comment, I'm wondering if I'm oversimplifying something that might actually be WAY complicated and difficult to learn!

That's proably rather easy to determine:
- A Sim from the Saved Sims folder (.sim file) has only 3 resource types SIME, SIMO and SNAP.
According to this list: http://simswiki.info/wiki.php?title...PackedFileTypes, SIME are the character data, SIMO the outfits (and genetics) and SNAP the thumbnail. Everything else doesn't really belong into a .sim file.
- A Sim that was installed as a sims3pack can also contain TXTC and an _XML file. Everything else doesn't belong there either.
- If a .sim file contains BONE, GEOM and other stuff then it probably contains custom content.

Nice link, thank you. So really there is no way to scroll through the files quickly to make sure there's nothing evil in there, huh? There are a lot of unknown file types that I thought were probably bad. Basically I'm going to have to do the modding tutorials to learn this process? Sigh