Short answers, pulled from the story.
A mandolin is a stringed instrument in the lute family, generally plucked with a pick, that most commonly has four courses of doubled strings for eight strings in total. It is tuned in perfect fifths to G3, D4, A4, and E5, the same tuning as a violin, and it is the soprano member of a family that includes the mandola, octave mandolin, mandocello, and mandobass.
The three most common types are the Neapolitan or round-backed mandolin, the archtop or carved-top mandolin, and the flat-backed mandolin. The round-backed version has a deep bowl built from wood strips, the archtop has an arched top and back carved from wood, and the flat-backed uses thin braced sheets of wood like a guitar.
The carved-top archtop mandolin is credited to Orville Gibson, an instrument maker in Kalamazoo, Michigan, who founded the Gibson Mandolin-Guitar Manufacturing Co., Limited in 1902. His designs evolved into the Florentine or F-style with a decorative scroll and points, and the pear-shaped A-style.
Mandolin notes decay to silence quickly, faster than larger instruments like the guitar, so players use tremolo, the rapid picking of one or more pairs of strings, to create sustained notes or chords. The instrument's paired strings make this easier because the pick can strike each string of a pair alternately for a fuller, more continuous sound.
Vivaldi wrote concertos for mandolinos, including one in C major, RV 425, and one for two mandolinos in G major, RV 532, while Beethoven wrote four small pieces in 1796 and enjoyed playing the instrument. Mozart's Don Giovanni of 1787 includes mandolin parts, and Mahler used the mandolin in his Symphony No. 7, Symphony No. 8, and Das Lied von der Erde.
Following its development in Italy the mandolin spread across Europe through mandolin orchestras, then took root in American country, old-time, bluegrass, and folk music after the early 20th-century mandolin craze faded by the 1930s. It is also played in classical Carnatic music in India, where U. Srinivas was perhaps its greatest player, and it saw renewed popularity in Celtic and British folk during the 2000s and 2010s.