Very brief introduction mostly for Mac OS X users,

The TeX wrapper is just a folder where one gathers everything related to a single document: text, images, macros, whatever you need.
It contains whatever you want, the only rule is the Document.texp folder which is designed to contain all the metadata.
By the way, any name with extension "texp" is acceptable.
On Mac OS X, it is not required for document.texd to be a bundle, it depends on the user experience.
But the Document.texp folder should definitely be a bundle because it only contains meta data for the frontends pupose only.

In the Info.plist file (an xml property list format widely used by Mac OS X), you will find general purpose meta data like the master document name and the file encodings, eol, language. An indirection table is used to manage properly the changes of file names.
The file *.spelling describes part of a spelling context: the list of ignored words which need not be the same for all the text files in the TeX wrapper.

In the frontends folder, different front ends will store their private metadata, in a file or a folder, whatever they want.
The names TeXShop was chosen for the presentation purpose, but nothing is yet defined concerning TeXShop.
Ideally, auctex should put its meta data in that folder too. The same for TeXniCenter and the others...

Maybe some confidential per user info might be interesting, this feature is copied from XCode.

The actual file specifications are available as part of the article I wrote for TUG 2004.

These specs are not closed in the sense that frontends must be very careful and smart in using them: they might be corrupted!