Unlike granite, diorite has no or very little quartz or alkali feldspar; unlike gabbro it is fairly light colored—half black and half white—and under the microscope it has sodic, not calcic plagioclase. If a dioritic rock is erupted from a volcano (that is, if it is extrusive), it cools into andesite lava.
The official definition of diorite is narrower than what the average geologist calls diorite. With more quartz, diorite becomes tonalite. With more alkali feldspar, diorite becomes monzonite. With more of both minerals, diorite becomes granodiorite. Maybe this will be clearer on the classification triangle.
Other galleries:
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society