When the developer of a shareware program decides to make it available via the Internet, he or she will find it inefficient to try to upload each individual program file to the Internet. . So they use some software to turn all the pieces of her program into a compressed archive . An archive is simply a single file that contains all of the information available in the individual program files. The advantage of an archive file is that only one file needs to be transferred over the network, both by the author to the anonymous ftp site, and by the ultimate user from that site. Compression refers to an operation that can take all the information in a file, and create a second, smaller file that can later be used to re-create the original file.
Files ending in .zip are archives of compressed files.
This allows a developer to create a single file that can be used to transfer a complete program over the Internet to an end user. If the file is a self-extracting archive, simply running it will uncompress it or run its 'setup' script. If the file has the .zip extension, you'll need a program that can 'unzip', or uncompress the files for you. If it has the extension .sit.hqx, it's a Macintosh 'stuffit'-ed file, and the Stuffit Expander can be found on the Software Downloads Page as well.