A metaphor is a comparison of two unalike things where one thing is said to be another thing. It is usually meant to emphasize some quality of the literal thing that actually exists in a new or unusual way by comparing it to the figurative thing (that is not actually there). One might say that the sunset paints the clouds with hues of pink and purple; this metaphor compares the sunset to actual paint (or even, perhaps, to a painter) to emphasize how bright the colors of the sky are. One might say that the sunset sinks beneath the sea's horizon, an orange dropped slowly into a blue bowl. Such a comparison, I think, emphasizes the color, shape, and speed of the sunset.

A metaphor simply means that you are describing something by saying that it is something else. For instance, I could say that "clouds are cotton candy." Although I could not physically eat the clouds, and they are not made out of sugar, the visual that this metaphor conjures up is one of a fluffy, puffy cloud. To describe a sunset using a metaphor, try thinking of things you think a sunset could look similar to, and state them as if the two things are equal. I could say that the sunset was a bonfire, or that the sunset was the dimming of the lights just before a movie starts. The first of these makes the reader think of a sunset that is bright and fiery, with red and orange colors. The second does not make the reader think of brightness or color, merely the gradual turning off of lights in a dark room.