They are cracks to get around copy protection so you don't need the original or in some cases, any CD in the drive to use the software. Some people feel they have legitimate uses if you bought the software, while others believe that even though some might use them responsibly, they are overwhelmingly used illegally in the ISO trading scene. (where copys of discs are traded that often are burned without the subchannel data needed to bypass Safedisc copyprotection and be seen as original discs)