Music theory is the study of the notes and symbols used when writing music and how they apply to playing. Generally, all instruments share much of the same concepts in music theory.
Documents on which music are written are called sheet music. Sheet music that contains music for more than one instrument are called scores.
Notes are written on five lines called a staff.
Both the violin and the fiddle use the Treble Clef, which is used to write notes on higher-sounding instruments. The lines of the treble clef are from bottom to top: E, G, B, D, F; the spaces are from bottom to top: F, A, C, E. You can remember both of those with the following chant: Every Girl Buys Dad Fudge, tells what's on the treble line. F, A, CE, tells what's in the treble space.
A time signature appears at the first line in every piece. It tells you how many beats are in a measure, and looks like a fraction; the top number tells you how many beats each measure has. Until a player steps up to the intermediate level, the bottom number will always be 4, and the longest measures will all have 4 beats.
The type of note tells you how many beats it has. For example, most notes are quarter notes, which have one beat. A quarter note is filled in black and has a stem.