This refers to conflicting elements that in some way deviate from the norm: a woman wears a long formal gown with running shoes and a sweatshirt.
Three forms are literalization, reversal, and exaggeration.
In literalization the joke comes from taking a figure of speech and then performing it literally. When Max Smart (GET SMART) asks the robot agent Hymie to "give me a hand," Hymie detaches a hand and gives it over, interpreting the instruction literally.
Reversal is simply reversing the normal, taking what is normal and expected and doing or saying the opposite. For instance, a parent typically takes care of his or her child. But if the child takes care of the parent, that's reversal.
An exaggeration is taking what is normal and blowing it out of proportion