They do this through the use of algorithms and coverting files into special compact formats. Although Winzip is the more popular of the two, I would recommend Stuff-It because its compression surpasses even Winzip. I have encountered one or two minor problems with Stuff-It Expander, while I have encountered none with Winzip. Try Stuff-It first, if it doesn't do what you want, use Winzip. Share your advice now .
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Another good idea is to compress your .jpg files on your website. I use Paint Shop Pro. When you save a .jpg it an option to set the compression level. Of course, the more it is compressed, the fast it loads and the lower the quality. Also, be careful if you open and save a .jpg with compression repeatedly, the quality will keep going down.