The Ukulele is a fretted, nylon-stringed instrument with historic roots in Hawaii. The name roughly translates to "jumping flea," the nickname of a court officer who lived in the early nineteenth century. A patron of musical arts, it was popularized by King Kalakaua.
The instrument is famous for its connections to Hawaiian music and culture, but also for its bright and gorgeous tone. I once had a friend who claimed it impossible to play a sad song on the Uke.
They come in several sizes including soprano, concert, tenor, and bass, although the first is by far the most common.
The Ukulele is one of my absolute favorite instruments to teach. Due to its small size and soft strings, fingers don't have to callus to play songs on the uke, so students may find themselves walking out of their first lesson with a full song under their belt. It doesn't take a whole lot of music theory to play and the fast progress that students can make on the instrument prevents them from being discouraged. I highly reccomend it as a first instrument to learn or for young students.