A mandola is a larger and lower-pitched version of a mandolin. As the name suggests, it is to the plucked string orchestra what a viola is to a regular orchestra.
There are actually 2 different pitches commonly used for mandola tuning. Some mandolas are tuned CGDA (which is the same tuning as a viola). However, most mandolin orchestra music is written for a larger version of the instrument which is tuned to a lower pitch (GDAE – like a mandolin but one octave lower). This is also known as an octave mandola. The MMO mandolists play octave mandolas.