Newsgroup posts are plain text. That means that all they contain are characters you find on the keyboard. There are also no special formatting commands like you get in a word processor like WordPerfect or Word (since those special formatting commands aren't plain test).
The Internet has always been used for file transfers and people figured USENET shouldn't be any different than any other portion of the Internet. So a method was invented for converting non-plain text (or "binary") files into plain text and then back again. You could then take a binary file like a picture or a sound and then run it through a program which converts (or "encodes") it to just a plain text file (usually increasing the file size from 10-50%). The new file is unusable as what it was, but it can now be sent in a USENET post.
There are several newsgroups dedicated to binary postings. Most can be found in the alt.binaries hierarchy. However, there are others that have "binaries" in the name which are also supposed to be for the posting of encoded binary files. Because of the size of even small encoded files, typically an encoded binary file should not be posted in a newsgroup not designated as a binaries group.
Additionally, some binary newsgroups have a second group of the same name but ending in ".d". With this setup, the first group is strictly for posting binaries and the one ending in .d is for regular posts (the "d" is for "discussion").
Once posted, the file contained in the article can be downloaded by anyone reading the newsgroup. (An encoded file in a post is usually called an "attachment", though it's a misnomer implying the the file is seperate from the post itself, which it isn't.) Once downloaded, the file can be run through another program to convert it back into its original binary form so it can be used as it usually would.
If a binary is extremely large, a newsreader will break the attachment into multiple messages. The newsreader usually numbers the articles in their subject. For example, the subject will have something like "(2/10)" in it. This means that particular post is the second part of 10 total posts. All ten must be retrieved to get the full file.
A Macintosh keeps track of what type of file it is as part of the file itself, i.e. there's a section of the file that tell the computer what program should be used to diplay it. Thus a file can be named anything, but the appropriate program should launch when the icon is double clicked on.
In the Microsoft Windows environment, the file's extension (what appears after the period in the name) tells windows what type of file it's supposed to be. A file ending in ".gif" is expected to be a GIF image and Windows will use whatever program is set up to display GIFs. If a file ends in ".gif", but is actually a WAV sound file, Windows will launch whatever program is used for GIFs and that program will probably complain that the file isn't a valid GIF image (which it isn't). However, if the file is renamed to end in ".wav", the associated WAV file player will be launched and then play the sound. The same thing can happen with files of the same class. That is, a GIF image could be inproperly labeled with the ".jpg" extension (which is for a JPEG image). If double clicked on, an image viewer will be launched and (even though the program may also display GIF images), it may complain about the file not being a valid JPEG image because it's trying to work with the file as a JPEG and not a GIF.
Another stumbling block for Windows are files that have no extension at all. Windows is then completely at a loss as to what type of file it is. It would be best to rename the file so that it has the appropriate extension.
Most binary files are not universal. For example a program for Windows will not directly work on a Mac. The same goes for Mac programs in Windows.
Some file types are slightly interchangable. For example, the Microsoft Word for the Macintosh has an option to read Word files for Windows (or "PC"). There's no real way to get Word for Windows to read Word for Mac's files. However, Word for Mac can be told to save the file as a Word for Windows file which can be read by the Windows version.
MIME
BinHex