My power cord for my computer and all usb cables i have seen have these cylindrical things on them. What do they do?
Those are called ferrites. You can learn more about them from a company called Fair-rite (I put their link down below). A ferrite is really nothing more than a magnet, but one specially tuned depending on the frequency of signal running down the cable, or the current if the cable is a power cable. Electro magnetic interference (EMI) is all around us, especially with all the electronic devices we have running in the modern home. EMI flies through the air and, when it contacts something metalic, it runs along that metalic line. In the case of a cable, imagine the clean signal picking up fuzz as it runs down the line. That's pretty much what happens. A ferrite stops the fuzz from getting through to the end of the cable, thus making the signal much cleaner when the end device gets to it. On a power cord, the ferrite helps prevent too much EMI from leaking out at the end and interfering with the device that the power is for. I should mention that a ferrite is always optional on a line, in other words the cable will work without one. However, ferrites aren't expensive and they do help with the signal, so most companies will just add one on to the product just in case.
Those devices are for supressing RF interference to external devices. The high data rates, tend to create signals just like transmitters, which can interfere, with you TV, cordless phone, stereo system, etc.
These cylindrical objects are there to filter out interference caused by electro-magnetic fields. They generally improve signal quality, and reduce noise in other circuits.
