This will automatically unstuff files compressed using most standard compression techniques. Zip is a common compression format in Windows. As well, you'll be able to create your own Stuffit archives for backup etc, and Drop Stuff is also accelerated for Power Mac Users. If you need to manually do this, simply drop the file onto the application icon. In addition to being compressed, files are usually converted to a format that actually slightly adds to their size so that there is some tolerance for inaccuracy in file transfers.
Many internet connections are standard telephone lines.
The redundancy added to the files allows a measure of error correction and detection that makes the internet a usable mechanism for file transfer. The most common Macintosh format to effect this is Binhex. This can potentially be more than one file when it is unstuffed. You don't need any special software to unstuff it. Just double click the icon you will see after you download the file, and the archive unstuffs itself. .sit means that the compression format is Stuffit, from Aladdin systems .