MIDI stands for music information data. It takes information transmitted by a controller, like a keyboard, and sends it to a an electronic sound generator.
USB is a data transmission standard for computers for periphials. There are currently two versions: 1.0 and 2.0. 1.0 supports 1.5 MBps and 2.0 supports 60 MBps in data transmission speeds.
MIDI is no where as fast as its new rival. However, it can connect older electronic music instruments and can connect multiple MIDI instruments directly to one another.
Depending on the version used, USB can be over 100 times faster than MIDI, enabling it to handle considerably more data. Currently, there's no way to interconnect multiple USB instruments directly to one another.
A way of integrating of USB with MIDI is through a MIDI interface. This is a hub into which you can plug multiple MIDI devices. The information for the MIDI interface is transmitted via USB to the computer.