Short answers, pulled from the story.
The viola is a bowed string instrument in the violin family, slightly larger than the violin and tuned a perfect fifth lower, with strings set to C3, G3, D4, and A4. Its body runs between 38 and 46 centimeters in length, compared to a standard violin body, and its heavier strings and larger size produce a darker, mellower tone. The viola bow weighs 70-74 grams, heavier than a violin bow at 58-61 grams.
Viola music primarily uses the alto clef, which places C4 (middle C) on the middle line of the staff. This clef is rarely used by other instruments. When the viola part moves into a substantially higher register, the notation switches to treble clef to keep the music readable.
The viola concertos by Béla Bartók, Paul Hindemith, Carl Stamitz, Georg Philipp Telemann, and William Walton are considered major works of the viola repertoire. Hindemith, who was a violist himself, wrote the concerto Der Schwanendreher. Bohuslav Martinů, Tōru Takemitsu, Tibor Serly, and Alfred Schnittke also wrote well-known viola concertos.
The International Viola Society (IVS) was founded in 1968 as the Viola-Forschungsgesellschaft and now has twelve chapters worldwide. The largest is the American Viola Society, which publishes the Journal of the American Viola Society and sponsors the Primrose International Viola Competition. An earlier attempt at an organization, the Violists' World Union, was established by Paul Hindemith and Vadim Borisovsky in 1927 but did not last.
Lionel Tertis and William Primrose were the pioneering soloists who most influenced the twentieth-century viola repertoire. Tertis had many major works written for or commissioned by him, including pieces by Arthur Bliss, Edwin York Bowen, Benjamin Dale, Frank Bridge, Ralph Vaughan Williams, and others. The Primrose International Viola Archive at Brigham Young University houses a major collection of scores, recordings, and instruments related to the viola.
The viola's four strings are tuned in perfect fifths from lowest to highest: C, G, D, A. Scordatura is an alternate tuning technique used in some classical works. Mozart specified in his Sinfonia Concertante for Violin, Viola and Orchestra in E that the violist raise all strings by a semitone, likely to give the viola a brighter tone against the orchestra. Tertis, in his transcription of the Elgar cello concerto, tuned the C string down to B in the slow movement to allow the viola to play one passage an octave lower.