The violin (or fiddle when used in the context of folk music) is a stringed musical instrument comprising 4 strings, each tuned a fifth apart from each other. The lowest string is a G just below middle C, then D, A and E (in that order).
Typically, sheet music for a violin uses a G clef (or treble clef) with the lowest attainable note the G just below middle C.
The highest note apparently available on a violin is all four fingers pressed down on the E-string (sounding a B). However this is only the highest note in 1st-position. A higher note can be achieved by sliding the hand up the neck of the violin and presssing the fingers down at this new position. Pressing the first finger on an F is called going in to 2nd-position. 3rd position is achieved when the first finger presses down on a G. Violinists often do this on the lower strings even though this seems unnecesary. This is done to produce a clearer tone or to handle a piece which would otherwise require fast switching of strings.
Double stopping is playing two or more strings simultaneously, producing a chord. This is much harder than normal single-string playing as more than one finger has to be coordinated on to different strings simultaneously. Sometimes going in to higher positions is necessary in order for it to be physically possible for the fingers to be placed in the correct places.
Vibrato is a very common device used by violinists, which causes the pitch of a note to vary up and down quickly. This is achieved by moving the finger pressing on the string slightly forwards and backwards. Vibrato can add much emotion to a piece, as well as disguising a misplaced note.
Pressing the finger very lightly on the string can create harmonics. This means that instead of the normal solid tone a wispy-sounding note of a higher pitch is heard.
Violins are tuned by twisting the pegs present in the head of a violin. The A-string is tuned first, and other strings tuned in comparison to it using double-stopping. Some violins also have adjustors. These can adjust the tension of the string and are positioned behind the bridge. These are more convenient when a not a lot of adjustment is necessary.
Strings are relatively cheap to buy and are usually replaced after about a year or when they break. It is said that Paganini purposefully weakened some of his strings so that in performance they would snap. He would then play the rest of the piece on the remaining strings, sometimes going in to impossibly high positions in order to impress the audience.
The strings of the bow are made usually out of horse-hairs. These have to be frequently rubbed with rosin (yes, that is spelt right) so they can gain enough grip on the metal strings of the violin.
The violin typically makes up the bulk of an orchestra with two sections: the first and second violins.
Note: AKA fiddle