How Files Are Compressed

• There are a variety of compression formats in use, but the basics are the same in each case. The compression software looks for patterns in each file and attempts to create a digital shorthand for them -- thus a row of twelve "A" characters might be replaced by "A12," though the actual techniques and algorithms used are much more complex than this. Some file formats lend themselves to compression better than others, so the level of compression will vary. Once a file has been unzipped, it is identical to the original.
  
  

Why Files Are Compressed

• Files are compressed for a number of reasons, including freeing space on a hard drive or building regular backups of data. Some compression programs include encryption and password-protection features, so zipping files can add an extra level of security as well. If multiple files need to be downloaded or transferred over the Web, compressing them into a single archive can make the task easier, which is why freeware downloads and digital music albums often come in a compressed archive format.
  
  

Compressed File Types

• Some file types automatically apply a variety of compression techniques without any assistance from third-party software. Common formats adopting this approach include JPEG images and MP3 audio files, and these formats are known as "lossy" because they don't retain all of the information included in the original -- though they're considered good enough for most users, their fidelity isn't perfect. As these files already use some form of compression, zipping them will not achieve much of a reduction in size. However, they can still be included in archives as normal. Files cannot be edited until they are uncompressed or extracted from the archive.
  
  

Compression Software

• The use of "zip" as verbal short-hand for the compression process originates from the DOS-based PKZIP and the Windows-based version WinZip, which both use .zip files as their primary archive format. One of the most widely used alternatives to WinZip is WinRAR, which can handle .zip files and also uses it's own .rar format for archives. Freeware alternatives include 7-Zip, which can handle both .zip and .rar formats as well as its own .7z archives. Basic archive features are built into Windows (from version XP and onward), including the ability to compress and uncompress folders.