A comparison of two things - without using the words like or as - is a metaphor. 

EXAMPLES of metaphor

You are my sunshine.

A wire is a road for electrons.

Example from the TV series House:  "Saying there appears to be some clotting is like saying there's a traffic jam ahead. Is it a ten-car pile up, or just a really slow bus in the center lane? And if it is a bus, is that bus thrombotic or embolic? I think I pushed the metaphor too far." (episode "Euphoria, Part 1")

"Words are bullets, and should be used sparingly, aimed toward a target." (Army Colonel Dick Hallock)

"All the world's a stage, And all the men and women merely players..." (Shakespeare)