The Simon task is a so-called “choice reaction task” (as compared to a “simple reaction task” such as a Go/No-Go task). In its most basic form it is a 2x2 design. Let’s suppose the relevant stimulus dimension for making a giving a response is shape: “if you see a circle, press left; if you see a square, press right”. The irrelevant stimulus dimension is location: whether the stimulus appears of the left or right side of the monitor. Thus, there are 2x2=4 combinations: 2 (shape: circle, square) x 2 (location: left, right). In a Simon task the irrelevant response dimension (perceiving the Stimulus on the left, right) overlaps with the response dimension (pressing left, right). This makes the Simon task highly suitable to study the interaction between perception & action. The relevant stimulus dimension (perceiving a circle or a square) does not overlap with the response dimension. Please note that the Simon effect occurs at the level of response selection (remember, the Simon task is a choice reaction task): the irrelevant location feature of the stimulus facilitates response selection when the features overlap (as when the circle appears left or the square right) but hinders response selection when the features do not overlap (as when the circle appears right or the square left).
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