The Mandolin is a cross between a violin and a guitar. It is tuned like a violin, having four notes, but each note has two strings which increases its volume. It has frets (thin metal cross bars) on the finger board, like a guitar. The first mandolins that we hear about are in Italy in the late 1600's. In the 1700's compositions like Vivaldi's Double Mandolin Concerto were being written for the mandolin. Handel included the mandolin in some of his music and so did Mozart. But suddenly, in the 1800's, the mandolin was forgotten.
Divertimento in F major (KV 138) by Wolfgang Amadeus Mozart performed by the Dutch Mandolin Chamber Orchestra
In the 1880's people in Germany and Austria began to re-discover the mandolin, and traveling Italian mandolin troupes began to travel in Europe. But an amazing thing happened: The mandolin also became very popular in Japan and the United States! Unlike the violin, the mandolin has no bow. It is simply plucked with a pick. During the 19th century, it became popular to pluck very fast back and forth using the pick to create a tremolo effect. The mandolin is tuned like a violin. Instead of 4 strings, however, it has 8. This means that there are 2 G strings, 2 D strings, 2 A strings, and 2 E strings. A piano can be quite loud because it has 2 or 3 strings for every note in order to create a richer sound. The same is true for the mandolin, it has two of each string to make it louder.
A wonderful 2-Disc set of Vivaldi's concertos featuring the "Concerto for Two Mandolins" here
Here's a great mandolin at a reasonable price, perfect for beginners.