The logarithmic scale is a nonlinear scale that is used when measurements differ by several orders of magnitude.
Have a look at following three functions plotted on a linear scale (left) and a logarithmic scale (right). Notice how the exponential function becomes a straight line on the logarithmic scale.

Common uses for a logarithmic scale include earthquake strength, sound loudness, light intensity,growth curves, and pH of solutions.
Check out thelogarithmic lawsto learn how to calculate with logarithms.