There are different kinds of sparks, but the spark you mean here is the kind associated with fire (as opposed to electricity). The sparks associated with fire are incandescent particles, which means they emit light because they are hot. These "fire sparks" are a special case of thermal radiation. Electrical sparks, on the other hand, are formed when an electric field creates a channel that is ionized and electrically conductive in the air.

Triboluminescence is a different phenomenon also: light is generated via the breaking of chemical bonds, as opposed to simple heating of the material, as is the case with "fire sparks." These chemical bonds break when some material is ripped, rubbed, scratched, crushed, or pulled apart, and the mechanism is not entirely understood. I hope this helps!