The phrase "all of the above" is used to denote a similarity between items in a list. This similarity may be in features, benefits, usage or desirability. The list need not be physically printed above the phrase. When used in this manner, in fact, the word "above" usually means "before" or "preceding" the phrase in either print or conversation.
For example, someone might state that he is eating a lot of meat, fish, eggs and peanuts and might explain that it is because all of the above are high in protein. In a printed piece extolling the benefits of eating a diet high in protein, the author might write a paragraph about meat, a paragraph about eggs, and so on. He might then conclude by stating that all of the categories mentioned in the preceding paragraphs are high in protein.
"All of the above" is often used in conversation as the answer to a question as well. For example, one man might ask another what he likes best about a new car: its style, its features, or its price. The second man might reply "all of the above," meaning that he values all aspects equally.
This phrase is also a common answer option on multiple choice questionnaires or tests that allow only one answer per question. For example, the question might be "which of the following has wheels?" The answer choices might be "a truck," "a bicycle," "a skateboard" and "all of the above." The correct answer would be "all" because selecting any other answer would imply that the other answers did not have wheels. If the test allows the taker to select more than one option, this phrase would probably not be listed as an answer.
The opposite of "all of the above" is "none of the above." This phrase can also be used to express a similarity, but it would express the lack of a similar feature rather than the presence of a similar feature; it could also indicate that offered choices all are undesirable as opposed to indicating that all are desirable. For example, if a server asks if a customer would like butter, sour cream, and cheese on his potato, he would say "all of the above" if he wants them all or "none of the above" if he'd prefer it plain.
Another related phrase is "any of the above." While "all" and "any of the above" are sometimes interchangeable, they can also represent very different answers. For example, if someone asks whether he should use standard pliers, needle-nose pliers or an adjustable wrench to turn a bolt, the answer would be "any of the above" because he is unlikely to use all of them at the same time.