A simile is a saying that compares one thing to another by using the words "like" or "as." For example, you could say "that person is as strong as an ox with muscles like solid steel" and you can get a picture of someone that's strong in your mind. Both of those compare two specific things so they're similes.
A metaphor is a saying that compares one thing to another but what makes it different is that a metaphor does not include the words "like" or "as.