The varieties of characters of representations of finitely generated groups into $SL(2,\mathbb{C})$ have interesting relations to knot theory, spectral geometry and geometric quantization, and their study goes back to the work of Poincaré on the monodromy of linear second order differential equations. We will survey some of the main developments in this theory, and show how the theory of conjugation invariants of $n$-tuples of 2 by 2 matrices can be used to clarify them.