In a typical computer system found in a home or office, you normally see these "bumps" on the mouse, keyboard and monitor cables. You can also find them on power supply wires when a device (like a printer or scanner) uses an external transformer. These "bumps" are called ferrite beads or sometimes ferrite chokes.
Their goal in life is to reduce EMI (electromagnetic interference) and RFI (radio-frequency interference). You can see these beads in the photo on to your right. A ferrite bead is simply a hollow bead or cylinder made of ferrite, which is a semi-magnetic substance made from iron oxide (rust) alloyed with other metals. It slips over the cable when the cable is made, or it can be snapped around the cable in two pieces after the cable is made. The bead is encased in plastic -- if you cut the plastic, all that you would find inside is a black metal cylinder.
Computers are fairly noisy devices. The motherboard inside the computer's case has an oscillator that is running at anywhere from 300 MHz to 1,000 MHz. The keyboard has its own processor and oscillator as well. The video card has its own oscillators to drive the monitor. All of these oscillators have the potential to broadcast radio signals at their given frequencies. Most of this interference can be eliminated by the cases around the motherboard and keyboard.
Another source of noise is the cables connecting the devices. These cables act as nice, long antennae for the signals they carry. They broadcast the signals quite efficiently. The signals they broadcast can interfere with radios and TVs. The cables can also receive signals and transmit them into the case, where they cause problems. A ferrite bead has the property of eliminating the broadcast signals. Essentially, it "chokes" the RFI transmission at that point on the cable -- this is why you find the beads at the ends of the cables. Instead of traveling down the cable and transmitting, the RFI signals turn into heat in the bead.