System Exclusive Messages
System Exclusive messages are used for more complex control and programming of synthesizers. Sysex can be used to control just about every facet of a synthesizer setup, for example, a single voice setup, a multi-voice setup or to change every voice on a synth at once. On some synths, they even allow you to perform the same functions as the control panel buttons. One problem with sysex is that it cannot usually be used in real time, that is, while a piece is playing. Receiving sysex often ties the synth up briefly causing a noticeable pause in the music.
Sysex tends to be hard to deal with. In fact there is a whole class of computer applications called patch librarians and editors to assist you in creating and storing sysex. These programs typically cater to one synth or one family of synths, and have a graphical user interface built around the voice architecture of the particular synth you are working with. This shields you from the underlying complexity of dealing with hex and binary arithmetic, checksums and other tricky stuff.