A simile is a figure of speech comparing two unlike things. You know it’s a simile because the word Like or as is used. (Her eyes are as green as emeralds. He can swim like a fish in water.)
A metaphor is a figure of speech comparing two unlike things without using like or as. (Her eyes were emeralds. He’s a fish in water.)
An idiom is an expression that does not mean literally what the words say. (He had a frog in his throat means that his voice is hoarse. The cat got her tongue means that she couldn’t speak.)
Definition: repetition of a the same letter or sound at the beginning of adjacent or closely connected words.
Definition: formation of a word by imitation of sound.