From CCRMA Wiki
Open Sound Control (OSC)
Open Sound Control in a protocol developed at CNMAT for controlling sound synthesis applications. It can be used simply as a replacement for midi with meaningful message names that are easy to parse out in Max/MSP or Pd. It can also be used to create a heirarchy of message that are bound for different receivers and provides support for discovering the messages an application knows how to respond to.
Open Sound Control Homepage
Open Sound Control Specification
MIDI - Music Instrument Digital Interface, is the result of a music industry effort to standardize the control communication between digital music instruments. MIDI in designed to be compact and easily parsed by machines (not humans!). MIDI messages are independant of timbre and describe only musical events such as note ons, note offs, and pitch bends. MIDI also provides message to change the setup of remote instruments via control change messages. Today MIDI is used everywhere from cell phone rings to controlling theatre lights and interactive art installations.
MIDI has some serious drawbacks in bandwidth/speed, resolution and reliability of computer interface hardware/software. While it's not the only solution to instrument - computer communication, it is still an appropritate choice for many applications.