It could be described as squeezing all the extra space out of the file. Compression saves disk space and transfer time when transferring files electronically. Compressed files must be uncompressed before they can be opened and used.
Some compression utilities can only uncompress one type of compressed file.
There are other compression utilities that can decompress many different types of compressed files. There are compression utilities available free for downloading. Norton Utilities also has a built-in unzip function. You will need to choose between a 32 bit or 16 bit version. If you aren't sure which to use, the 16 bit will work on any PC with Windows.