The bass and treble controls are essentially a 2 band equalizer. The bass control is usually set to vary the level of audio in a band of frequencies centered around 100 Hertz. The treble control acts similarly but its band is centered around 10,000 Hertz. There are a couple of different kinds of 'curves'. The following diagram shows a 'shelving' tone control. You can see how there is a wide range of frequencies where the boost is constant until you reach the 'slope' of the curve. Most head units claim to vary the level by plus or minus 10dB (as is shown in the diagram). In reality the range may be only +/-7dB or as high as +/-12dB. As the level of the control is increased or decreased the band of affected frequencies generally widens.

In this diagram, the red section is the response curve at 0 to +10dB boost (in 2dB increments) for the bass control. The blue is the same for the bass cut. The yellow is treble boost and the green is the treble cut. As you can see, this type of curve covers a lot of frequencies at full boost or cut.

This next diagram shows another boost/cut response curve that's not as common as the first but is used on some equipment. Its curves are similar to those that you'd get with an equalizer.