A mandolin is tuned the same way as a violin: G, D, A, E. But rather than having just 4 strings like a violin, it has 8 strings, arranged in 4 pairs, which gives it a distinctive sound. It is plucked with a plectrum, and can be used to play melody or chords, though it's main job tends to be melody.
Mandolins come in two basic shapes: bowl-backed (the traditional form of the instrument, from Italy) and flat-backed (a shape which developed later in the USA). If you are a beginner you will certainly get on better with a flatback – they are much easier to hold and more versatile musically. If you have a particular interest in the classical side though, you will want to try a good bowl-back at some point.
There are also larger, lower pitched versions of the mandolin, called "mandola" and "octave mandolin".
A set of folk tunes on a flatback mandolin (Simon Mayor)
A classical mandolin performance (Chris Aquavella)
The complete mandolin family in action (Modern Mandolin Quartet)
For more information about mandolins try these links:
Probably the most visited mandolin site in the world – a must for any enthusiast whatever your interests and style, and very well linked.Simon Mayor
Simon is probably the best known exponent of the mandolin in the UK. His recordings and tutor books are inspirational and lots of fun – as are his live performances! His site also has an extensive mandolin link page.BMG (Banjo Mandolin and Guitar Federation)
A national organisation devoted to plucked string instruments. The BMG organises regular workshops, summer schools, competitions etc. and most of the mandolin groups and orchestras in the country are affiliated. Members get a very good quarterly magazine as well.