Similes are used to compare 2 things using "like" or "as". NOT THIS- "I like reading." A simile IS this- "The ice cream is as cold as the winter snow." Or it could be "You are like a dog, loyal to your owner." A simile is NOT this- "She called me as I walked out the door." A simile IS this- "That person is as cool as an ice cube."
Metaphors are used to make the sentence bold and strong. A metaphor is comparing 2 things without using like or as. NOT THIS- "The dog is a fast runner." You can use the word is or anything else (except like or as), but you can't say "The dog is a fast runner." Why? Let me explain, you have to compare the subject with something that they are NOT (you need to find something that they are not, but something that you can compare the subject with). Saying that the dog is a fast runner is not right. Is fast runner a noun? No. So you cant have that (you have to make sure that the thing you are comparing the subject with is a noun). But, saying "The dog is a speedy rocket." IS a metaphor. When using metaphors that are not obvious, it is good to explain the metaphor after using it. such as "My Dog is a rock. She is very strong." or "The summer sun is an oven. It is very hot."