Transistors operate extremely well in amplifing small signals. They're not actually a switch per se, but can be used as one. Basically, the transistor watches the current into it's base terminal, and will output a proportionally scaled (ish) current through it's collector. The ish bit is because the signal isn't totally linear, but follows an interesting curve. This is why some amplifiers have different classes (like A, AB, B, C, D and H), as they have different methods for coping with the non-lineararity of transistors.