Music Notation for the Violin
All music for the violin is written in the treble clef. The standard
tuning for the open strings is as follows:
The violin has a range of 2 octaves + a M3 in first position. This is the range that is used in beginning (elementary) level repertoire. As the player learns to shift, the range expands higher. Much of the music for orchestra written in the 18th century did not require shifting past 4th position (intermediate range below), but solo and chamber music repertoire would often make greater demands on the player. By the 19th century, many composers were making greater demands in their music. Playing standard orchestral repertoire of the 19th and 20th centuries requires the ability to play the advanced range indicated below (please note that my choice of C as the upper limit is approximate - many scores do not require playing that high, others require playing higher).
The graded systems that are in use by many publishers of educational sheet music (and many state arts organizations, ASTA, MENC, etc) base the difficulty level of the music in part on the range demands put on the player. The Beginning Range above corresponds to Grades 1 and 2; Intermediate to Grades 3 and 4. Advanced-Intermediate range may be considered shifting into 5th and 6th position (ability to play 3 octaves above the low G string). Anyone aspiring to play the standard orchestral repertoire on violin should be comfortable playing in the high positions.
There are many factors to consider in addition to range when considering the difficulty of a piece of music including key, meter, rhythmic complexity, bowing complexity, independence of parts, use of double stops, shifting requirements, dynamcs, phrasing, and length. Anyone who is considering composing or arranging for strings should spend some time learning to play the instruments to truly understand the requirements for idiomatic writing, and should spend time studying orchestration.