In your computer mouse cable, or USB device or monitor cable, often there's one big ugly chunk of cylinder somewhere in the cable. What's that big ugly cylinder for?
Answer: It's a electronic component called Ferrite bead. It's used to suppress high frequency noise in electronic circuits. This is needed because cables that are not Shielded can also act as antenna and broadcasting device, creating inteference.
Why Apple's computer doesn't have such ugly bump? I guess because either Apple uses shielded cables, or the component is moved to inside the computer.
Thanks to Aaron Coakley for answer.