What are the plastic lumps that are on electronic wires (usb for instance)? Whats is their purpose?

Answer 1

they are called ferrite beads and are there to reduce EMI. Its sometimes needed in order to meet FCC testing.

Answer 2

They are ferrite beads. A long wire or cord can act as a radio antenna and pick up strong local radio stations which will then travel into your computer causing interference. One or more ferrite beads wraped arround a wire will choke off the flow of the interfering radio-frequency current.