There are a huge variety of Tiples. 'Tiple' (apparently pronounced 'teh-pleh') means 'treble' in Spanish, so can be applied to any instrument (there are tiple guitars and banjos), but the Martin tiple is very much a tiple ukulele.
Tiples have ten strings. They are divided up into four courses and tuned like a ukulele (usually in D-tuning). There are two A strings: one a high A and one an octave lower; three D strings, the middle one being an octave lower; three F# strings, again the middle one is an octave lower; and two B strings in unison.