Violins make sound by dragging the bow (stretched horse hair) over a string, causing the string to vibrate. At the micro level, the bow pulls the string to one side using friction, until the tension on the string pulls it back - this happens hundreds of times per second, and forms the basis of the sound we hear.
Horse hair is slippery by default. To create the necessary tension violinists apply a small amount of solidified tree resin by wiping a piece along the length of the bow. This piece of hardened resin has the same approximate texture as glass or hard candy, and is called rosin.
No. It comes in pucks or small blocks and you simply draw the bow across it or pass the rosin up and down the length of the bow a few times and that's usually enough. Too much rosin can gum up the bow.
Originally? Probably lack of options. These days the aim of the game is sounding "like a violin", so naturally there's very little innovation in violin technology.
