You'll always see the two different kinds of rosin. Dark, and light. But what's the difference?
Basically, rosin is the most important part of your set up, and also the least expensive. It makes the hairs on your bow sticky so they can grasp the strings when you slide it across. You could compare it to fishermen all hooking some fish at the same time.
Light: Some argue that light rosin is the best choice for treble instruments. It's harder and doesn't melt as easy.
Dark: Most people say this is softer, more likely to melt, and has a better grip. Sometimes you get a light rosin with a dark color added.
But there are other things that go into picking the best rosin for your instrument. Where you live and your climate could change what you should use. Your type of strings and brand of rosin may or may not mix well together. Some rosins can increase the build-up you get making you have to clean it more - although you should wipe it after you play every time.
One brand of rosin compared to another can completely change the tone of your instrument.
Also, another factor... whether your bow is made of synthetic hair, or horse hair. Horse hair is always the best for playing.
