An analogy is an implied (unstated) relationship between two pairs of objects. Because the relationship is implied, the first thing you must decide is what kind of relationship exists between each pair (i.e., synonyms, antonyms, homonyms, part-to-whole, time, place, age, etc.). This relationship can be any of the associations you have already studied.
An analogy deals with the relationship between two pairs of words. The two pairs are usually dissimilar (not the same), although the relationships between each pair are similar. Because analogies require you to identify similar relationships between dissimilar objects, understanding analogies is one of the highest levels of thinking.
Analogies are usually written in one of two forms:
Up is to down as fast is to slow
Up : Down :: Fast : Slow
The colon (:) in the second example means "is to." The pair of colons (::) means "as."