Like the guitar, the mandola is a poorly sustaining instrument --- a note cannot be maintained for an arbitrary time as with a viola.
Similar instruments are the mandolin, the octave mandolin (an octave below the mandolin), and the mandocello (octave mandola), which is tuned an octave below the mandola (like a cello). All of these have 8 strings tuned in unison.
Mandolas are commonly used in Irish music and folk music.