Answer:
"A metaphor is a figure of speech that pulls comparisons between two unrelated ideas." - Masooma Memon
For example: His words cut deeper than a knife.
It's different from a simile because a simile usually uses "like" or "as" instead of just being more... straightforward about it (if that's the right way to describe it) like metaphors do. Metaphors say that two things are similar not literally, but figuratively and poetically.
Example of simile: You are like a summer's day.
If this was said like a metaphor, it'd be: You are a summer's day.