In typical use Memory Stick is used as a storage media on a portable device, and the contents are then transferred to a computer. Since Sony is a major copyright holder in the music industury, they later added an encryption system, MagicGate, in order to block moving content between more than one computer.
For example, a digital camera will store images onto a memory stick. The memory stick may be removed and plugged into a computer or printer with an appropriate slot so the images may be transferred to the computer or printed. However if the content was written to the Memory Stick using software aware of MagicGate (like music "burning" software), that content cannot then be moved to another machine.
Memory Stick is essentially identical to MMC, both internally and in most dimensions except length. Given MMC's widespread use even when it was being introduced, Memory Stick's limited market appeal should not have been surprising. The reasons for creating a new largely-identical standard in the first place appear unclear, and since readers are built into few machines except those from Sony, many have compared it to Sony's early failed attempts to create standards, like Betamax. Why then they further clouded the issue with Duo is even more difficult to understand.
See also : removable media