What Happens When Files Are Zipped?by Jeffrey Ober
The term "zip" actually means compress. When you read the words zip, zipping, or anything relating to the z in computers, it nearly always means compress the file, or make it smaller. If you are using the Windows operating system, you will seldom see the term "zip" and instead will see the word "compress." For example, if you right-click on a file in the computer explorer, you will see an option to "Send To...Compressed" folder. There are dozens of programs and different ways to compress or zip files from utilities contained in the operating system to free utilities that attempt to find better ways to compress the information stored in the file.
A file is compressed so that the file on the disk or other media takes up less space. Compressing doesn't do anything else to the file, it just attempts to make the file smaller. This is important if you have a disk that doesn't have a lot of space. The two largest reasons that files are compressed are to send them over a network and to make a backup. When you send files over a network, the size of the file will determine the amount of time it takes the file to get where it is going. The smaller the file, the faster it can be transferred. When making backups, especially regular backups, disk space can be at a premium and the files seldom need to be accessed, so compressed files make the storage of backup information much more efficient.
The zipping process must keep all the data intact. After a file has been compressed, it will not be readable by any programs other than another file decompression program. However, the decompression program will be able to restore the file to its original size and composition. Different utilities use different processes to compress files. File compression algorithms can be very complicated, but the basic process is that the compression program looks for repetition in the file and replaces the repeated characters with symbols that indicate how many and what types of repetition have occurred. For example, the number zero repeated 100 times might be replaced with 100x0 to indicate 100 zeros.
Compressed File Sizes
Some files will compress a large amount, while other files will only be a slight bit smaller after being compressed. This is all based on the algorithm used by the compression program. In fact, the same file compressed in different ways can actually end up with a different compressed file size. Often, text files and photographs allow the most compression because they have the greatest likelihood of having the repeated characters and symbols.