Since the first historic eruption was recorded by the Spanish in the 16th century, the volcanism has been lightly explosive, producing ash clouds and pyroclatic flows in what can be described as vulcanian to sub-plinian events.
This volcano became infamous in 1993 when it sadly killed nine people (six of who were scientists) who were in the summit crater at the time of the eruption. Prior to this sad event, Galeras was designated a Decade Volcano in 1991.
Today is the 20th anniversary of the eruption of Pinatubo in 1991. Over at the Eruptions blog by Erik Klemmeti there is an excellent anniversary post, I reccomend you check it out!