A simile is a figure of speech in which the subject is compared to another subject, for example, "as nervous as a long-tailed cat in a room full of rocking chairs". Frequently, similes are marked by use of the words like or as, "The snow was like a blanket". However, "The snow blanketed the earth" is also a simile and not a metaphor because the verb blanketed is a shortened form, of the phrase covered like a blanket.
The phrase "The snow was a blanket over the earth" is the metaphor in this case. Metaphors differ from similes in that the two objects are not compared, but treated as identical, "We are but a moment's sunlight, fading in the grass."
See also tertium comparationis.