Alliteration is the repetition of a single letter in the alphabet (as in "Peter Piper picked a peck of pickle peppers.") or a combination of letters (as in "She sells seashells by the seashore."). It's just about the easiest form of repetition a poet can use.
A metaphor compares two unlike things. "My baby sister's a doll," you might say, compares your sister's size and sweetness to that of the perfection of a doll. At another time you might say, "My brother is a rat." This compares your brother to the nastiest little creature you can think of. In both cases you would be making a metaphor - a form of comparison that directly compares two unlike things. A metaphor wastes no time in getting to the point.
If you said, "My sister is like a doll," or maybe, "My brother's good as gold," you would be making a simile - a form of comparison in which one thing is compared to another unlike thing by using specific words of comparison like like, as, and resembles. Poets try to find unusual metaphors and similes.
In its simplest form, onomatopoeia is produced by a single word that sounds like the thing it refers to: "Six burgers were sizzling on the grill." "A snake slithered through the grass."
One of the most familiar kinds of comparison is personification---that is, speaking of something that is not human as if it had human abilities and human reactions.
A great exaggeration used to emphasize a point, and is used for expressive or comic effect. A hyperbole is not to be taken literally. Example: "An apple a day keeps the doctor away." We know that eating an apple every day will not keep you from ever getting sick and having to go to the doctor.
Idioms are groups of words whose meaning is different from the ordinary meaning of the words. The context can help you understand what an idiom means. For example: "Put a lid on it." Our teacher tells us to put a lid on it. She's not really telling us to put a lid on something but to be quiet and pay attention.