If you've downloaded software from the Internet or CompuServe's file libraries, you've probably seen them -- compressed files. Those are the files with funky extensions like .ZIP and .Z, .CPT and .GZ. After you've downloaded one, your computer can't use it until you've decompressed it. It's not much of a hassle, really, but you've got to know the tricks of the trade.
Why are compressed files even necessary, anyway? For two reasons. First, to make the files smaller. Compression squeezes files down so they take up less disk space -- and, as a result, take less time to transfer over a modem. Just how much smaller the files are depend on the compression method used and the type of information being compressed -- text compresses very well, but some image file formats (such as JPG) don't compress very much.
Compression provides another benefit: the ability to archive several files together into one file. Imagine you want to download the shareware game Quake, which is comprised of about a bazillion little files -- levels, sounds, documentation and so on. It would be a drag to have to download each file individually -- but Quake is distributed as a single, compressed, file. Just download it, and when you uncompress it, it will expand back into a bazillion files.
For intrepid Internet users like us, the trick is knowing how to identify compressed files and how to uncompress them. There are only a handful (well, a big handful) of popular compression formats that you'll find on the Internet.
For DOS and Windows, the most popular compression method is .ZIP. You can tell a file has been ZIPped because it has a filename extension of .ZIP (easy!) An older compression system that you will occasionally see is .ARC, and a newer one that is gaining popularity is GnuZip (.GZ). A great shareware program that you can use to uncompress these and other compression formats is PKZIP. (http://www.pkware.com/download.html) I'm also fond of the free program Stuffit Expander for Windows, which can uncompress ZIP and GZ files as well as files compressed by Mac users. (http://www.aladdinsys.com/consumer/expander1.html)
Speaking of Mac users, Stuffit Expander is the program you'll need to handle files compressed on a Mac. The most common compression formats on the Mac are .SIT (StuffIt) and .CPT (Compact Pro archives). Expander can also gracefully deal with .ZIP, .GZ and popular formats from other platforms. You can get StuffIt Expander from http://www.aladdinsys.com/consumer/expander1.html -- it's an essential, and free, tool.
If you come across a file with the extension .SEA (on the Mac) or .EXE (for PCs) then you've found a "self-extracting archive". These are programs that, when run, automatically decompress themselves. The good news here is that you don't need to think about which program created the archive, since you won't need a special program to uncompress it. The bad news is that the code to make an archive self-extracting adds 15 to 20K to the size of a regular compressed file, so they take a little longer to download.
You'll find other compression file formats floating around the Internet. The .Z format is popular among users of the Unix operating system, as is .tar (Unix "tape archive" files, which isn't technically compression at all, but file archiving.) Unix users also use the GnuZip compression method (.GZ).
Remember: just because you can uncompress a file doesn't necessarily mean that you can use it on your computer. If you have a PC, you can certainly download a Mac game compressed with the .SIT format. And you can even uncompress it with StuffIt Expander -- but that won't magically make that Mac program work on your PC. But, should you download a graphics or sound file that was compressed on a different operating system, you can see or hear it if you've got the right kind of viewer: for instance, a Mac-compressed QuickTime movie will work on your Windows PC if you have a QuickTime player.
A good decompression program (along with a good virus-checking utility) is an essential tool for everyone who downloads software from the Internet. Setting it up is a one-time task: download it, install it (uncompression programs come as self-extracting archives) then the next time you've got a compressed file on your hands, you'll be able to uncompress it and use it right away. After you've decompressed it and are sure the program works, you can delete the compressed version to save disk space.
=*=*= SITES MENTIONED IN THIS ISSUE =*=*=
PKZIP for DOS and Windows: http://www.pkware.com/download.html
StuffIt Expander for Mac and Windows: http://www.aladdinsys.com/consumer/expander1.html