Universal Serial Bus (USB) is a connection input and output used to connect various devices to a computer without the need of various special cables or equipment.
The Musical Instrument Digital Interface (MIDI) is a method of connecting an electronic instrument to other electronic equipment.
USB to MIDI cables are used to connect MIDI devices to a computer that does not have a MIDI input, such as a laptop. These cables usually do not require any additional software, and are relatively easy to connect and use.