Sheet music
Sheet music can be published as a separate copy for one piece or song, or it can be a collection of pieces in a larger book. If there is a piece of music for violin and piano, then the pianist will play from the piano part, which will have the piano music written, as usual, on two staves.  Above those two staves, printed slightly smaller, will be another stave with the violin part.  The violinist will play from a separate part which will just have the violin music.  This means that the pianist can always see what the violin should be playing, but the violinist will not be able to see what the pianist is playing.  The violinist may need to look at the piano part sometimes to see how the violin and piano part fit together.  It would be difficult for the violinist to play from music with both parts because he would need to turn the pages too often.  (Unless the violinist had a few measures of rest right before the page turn, the violinist would miss playing some of his/her part while turning the page.)