Homer's epics employ extended similes, such as this passage from the Iliad:
As a mountain snake, who is maddened by the poisonous herbs he has swallowed, allows a man to come up to the lair where he lies coiled, and watches him with a baleful glitter in his eye, Hector stood firm and unflinching.Later epic poets have imitated this trait so often that such long similes are called "epic similes."
See also metaphor, where the comparison is only implied (i.e., without "like" or "as").
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.