Similes and metaphors are both figures of speech. They are ways of describing an object by comparing it to something else.
A simile uses the words 'like' or 'as....as'.
His feet are as big as boats.
A metaphor states that one thing is something else. It is a comparison, but it does not use 'like' or 'as....as' to make the comparison.
His feet are boats.
In stating that 'his feet are boats', we mean that his feet are very big.