This task, considered the gold standard of attentional control, assesses a number of different executive functions. In this task, an individual is presented with a list of words printed in different colors, and must identify the color in which a word is printed, while ignoring the meaning of the word (i.e., say “blue” when reading the word “red” that is printed in blue ink). Both behavioral and neuroimaging data suggest that multiple functions are assessed by this task. In particular, it appears that one aspect of the task requires the top-down internal control over behavior, which is required to select the stimulus-attribute on which behavior should be based (i.e., on the basis of the ink color), rather than having the response being driven by salient distracting information (i.e., the word). There is also evidence that this task taps the ability to inhibit or override a prepotent response.
Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology. 18 (6), 643-662.