I have often heard that Paganini's music is considered very technical. What does this exactly mean? What about music makes it technical. Is music that is technical just difficult or can there be non technical music that is also difficult?
"Technical" means that it requires an impressive amount of sheer technique (that is, muscle control) to play. Paganini frequently pressed the possibilities of the violin to the extreme. His music includes very fast runs and arpeggios that require extremely skilled fingering and bowing, complex sequences of double stops, harmonics, advanced bowing techniques, etc.
In most cases, difficulty is due to technical demands, but there are other reasons that music can be difficult. Debussy has some slow piano music that isn't technically difficult, but requires an immense amount of touch and thought to be performed well. Another example could be the first movement of Moonlight Sonata: very technically easy, but very boring to listen to unless it's interpreted well. I recently performed The Rite of Spring, and while my part was not very technically difficult, the piece is so massive and emotional that it was difficult to stay focused for the entire performance.
Paganini wrote some very hard to play pieces that are not all that musically interesting. That's sort of the antithesis to Bach's Solo Partitas and Sonatas for Violin which are pretty hard but at the same time quite musically rewarding.
Basically, when listening to Paganini pieces, I think the player deserves better music, when listening to Bach, I think the music deserves better players.