A metaphor is a figure of speech in which there is an indirect comparison or contrast between some words, without using 'like' or 'as'. These words always have something in common in their meaning.
A metaphor expresses unfamiliar (the tenor) in terms of the familiar (the vehicle). The tenor is equal to the vehicle, formula: [A is B].
Metaphors can be divided in two ways - semantically and structurally.
Semantically: original (E.g .It's been a purple dinosaur of a day) and trite (E.g. A blanket of snow covered the garden).
Structurally: simple (E.g. The UK is a melting pot of cultures), prolonged (E.g. It will take a big tractor to plow the fertile fields of his mind. ), mixed (E.g. It was playing with fire in the belly.)
"Metaphor is a device for seeing something in terms of something else. It brings out the thisness of a that, or the thatness of a this." (Kenneth Burke, A Grammar of Motives, 1945)
An interesting video about our topic: http://www.youtube.com/watch?v=eFgRj-5d5Ac&feature=player_detailpage