The theorbo is basicaly a lute on steroids. Lutes are the pear-shaped thingys that abound in Renaissance music and artwork, similar to a guitar in appearance, but very (very) different in construction. Lutes have 14+ strings, each course is double-strung, though (except the highest one) -- like a 12-string guitar, I believe. So anyway, at the beginning of the 17th century, as music started simplifying into just a melody and chords instead of polyphony, this monstrosity was created. It's a lute with seven bass strings added an octave below, one for each note, so it ended up being about six feet long. The bass strings are unfretted, and used for extra emphasis on chords and whatnot.