In a linear taper pot the resistance is distributed evenly, so if you set a 10K pot in the middle you would have 5K on each side of the wiper, set it 3/4 of the way you'd have 2.5K on one side and 7.5K on the other.
In an audio taper pot the resistance is not even across it, it is related to the logarithm of the wiper position. This is because of the way our ears respond to sound, they can detect smaller changes at low volume levels than at high. As you turn an audio pot up, the resitance changes slowly at first, and then as the setting gets higher the resitance changes faster.
Here's a chart of an audio taper pot http://media.maxim-ic.com/images/app.../A254Fig01.gif
Note how at the halfway point, the resistance is divided around 15/85 instead of the 50/50 it would be in a linear pot.