Metaphor
A Metaphor is a figurative use of language that is used to paint one concept with the attributes normally associated with another.
For example, "ship of state" is a metaphor that likens the government to a ship: just as a ship needs a captain to make decisions, give orders, and control things, so a government needs someone to make decisions, give orders, and control things. By refering to the "ship of state" one emphasizes this aspect of government.
Metaphors are very powerful tools because they allow for the expression of very abstract principles by reference to concretes. They can also be dangerous to understanding, in that people may fail to recognize the figurative nature of a metaphor, and come to take it literally.
Many consider metaphor to be at the heart of poetry (or even to define in part what it means to be human): the figure of speech that links dissimilar objects for their resemblance. For example, Emily Dickinson uses "the white assassin" as a metaphor for frost. Ground may have a blanket of snow where blanket is a metaphor for cover.
As a subject, metaphor is as complex and deep as one likes. Those interested in further exploration might cosider Julian Jaynes, "The Origins of Consciousness and the Bicameral Mind."