simile is from the Latin word for 'like'. It's a figure of speech expressing the resemblance of one thing to another of a different category, usually introduced by 'as ' or 'like'
Simile 'In battle,John was like a lion', 'The wrestler is like a bull '
Metaphor is from Greek for 'transfer' In metaphor we transfer the image or meaning of one thing to another.It's a figure of speech where we treat two different things as the same. Instead of having something expressed as being like another thing, we say it is that thing.
So, the two similes above become metaphors by saying "In battle, John WAS a lion" and 'The wrestler IS a bull '. We don't mean either of those literally, but each is an effective image.