Most mandolins, and almost all F-style and A-style models, feature eight strings tuned in tightly spaced pairs, which makes it feel like playing a four-stringed instrument. Its small, shallow body and high pitch means that sustain isn’t a mandolin strongpoint, but to some extent this can be overcome by rapidly picking a pair of strings, a technique that’s spawned that signature mandolin sound.
Because the four courses of strings are tuned to the same notes as a violin it’s easy to pick out fiddle tunes, which explains why the mandolin sound has become a much-loved part of US folk and bluegrass.