For example, an Iowa district court in November 2006 halted all sales of See Clearly Method kits that had been marketed for several years by a company called Vision Improvement Technologies as a way to improve vision through eye exercises.
Jailbreaking is a hack that allows developers to access the file system of your iPhone, iPad, iPod Touch, meaning the root directory. In simpler terms it means that developers can now have access to all the files on your iPhone, iPod Touch, iPad meaning they can create apps that are possible of doing pretty much anything.
Jailbreaking an iPhone is the sort of thing done by tinkerers, and isn't advised for the average joe. Put aside the moral arguments about who owns what (and who should pay for what) and you're left with the practical advantages of jailbreaking - being able to install blocked software and replace key services - with the disadvantages of jailbreaking - having a less secure system that's more error-prone.