A simile is a comparison between two dissimilar things using the words "like," "as," or "than." It is used to illustrate a similarity between the two concepts thus compared.
"This room is like an oven."
"The crowd was as noisy as a herd of elephants."
"This is more fun than a barrel of monkeys."
Simile differs from metaphor in the use of the comparative word. Whereas a simile establishes two distinct but similar concepts, a metaphor asserts that the two concepts are, in some sense, the same. Thus, "He's like a juggernaut on the football field" is simile; "He is a juggernaut" would be the metaphorical equivalent.