The Comparison node offers you the ability to use a logical comparison operator as a way to combine textures. The node takes five inputs: the first two inputs are the textures for comparison. The next input is for the comparison, and the last two inputs are the result of the comparison. In the following example a Checks texture node (input A) is used to compare the black and white of the checks texture against white (input B) where the black areas of the checks texture are less than input B which is white. If the statement is true (black areas), those areas will be replaced with Red. If the statement is false (white areas), those areas will be replaced with the Fractal Noise (figure 1).
Figure 1: A Checks texture node is used to colorize the checks pattern with red and Fractal Noise using the Comparison node