JPGRAR's are a clever application of pseudo-steganography that enable you to embed a .rar archive (or .zip or .7z sometimes) in a .jpg picture (or .png or .gif if you're a masochist). The concept is simple: the data from a .rar is appended to a .jpg file, picture viewers ignore the extra data at the end and good archive programs (WinRAR and 7-zip) ignore the JPEG data at the beginning. To open it, all the user has to do is change the file extension from .jpg to .rar and open normally. A good way to tell that a picture is a jpgrar is by examining its filesize. If the size is 200k+ on a small or medium .jpg, there's a good chance that it's a jpgrar. Currently The Google Bordello has no unmarked jpgrar's, but look for some in the future as we find content to distribute.

How does it happen? This handy graphic from the internet (probably from some kind of chan) has ALL THE INFORMATION

Yeah. You make it just by combining two files with the dos copy /b. Some say that the .jpg has to be smaller than the .rar, which is a good rule to follow anyway. Here's jpgmerge version 0.04, a simple batch file that'll do all that for you. It's in the following jpgrar:

Jpgrar's have been around for some time, but their exact origin can't be confirmed. Some claim that they were invented by Usenet users, much like the XCD format, to solve a uniquely usenet problem. Because some usenet servers would only allow users to post pictures or text files, usenet posters made jpgrar's, or simply renamed whole rar's to .jpg files. Some say that jpgrar's were invented by 4chan or one of its offshoots. Image boards often don't check pictures for excess data, and so jpgrar's can be used to sneak files onto image boards. A good example of this is Bookchan, a group that distributes e-books via jpgrars on AnonIB. Because of the nature of e-book trading, jpgrars have a number of advantages over traditional ebook distribution methods, even when working outside the constraints of an imageboard. Often, ebooks are difficult to catalog, and present an unattractive package to viewers. Though jpgrar's often eliminate important information (particularly text type and proofing number, though these could be better contained inside the archive or text itself), they're easy to sort and able to be identified immediately. They have the same advantage that a product with pretty packaging has in real-life stores.

See also: Google Cache and the UUEncoded Exploits (an unpublished Tek Jansen adventure).

UPDATE: Google Wave has been confirmed to support embedded JPGrar's as of December 2009.