An analogy is a literary device that helps to establish a relationship based on similarities between two concepts or ideas. By using an analogy we can convey a new idea by using the blueprint of an old one as a basis for understanding. With a mental linkage between the two, one can create understanding regarding the new concept in a simple and succinct manner.
Example:
In the same way as one cannot have the rainbow without the rain, one cannot achieve success and riches without hard work.