Similies are comparisons that use 'as' or 'like'.
Cold as ice
Bold as brass
Quiet as a mouse
Sleep like a log
Metaphors, unlike similies, do not describe things as or like other things. Instead, they describe things as though they actually were something else.
"He's a pig"
"I fell into a sea of grief"
If a metaphor is sustained over a section of writing, it becomes an extended metaphor.
For instance in Carol Ann Duffy's poem 'Valentine', an onion is used as an extended metaphor for love:
"I give you an onion.
It is a moon wrapped in brown paper.
It promises light
like the careful __dressing of love.
It will blind you with tears"
Many metaphors are used so frequently in everyday language, that they are no longer really thought of as metaphors. Expressions such as these are known as 'dead metaphors'
"She's very laid back"
"He's living on borrowed time"
"I'm on top of the world"
"The eye of a storm"
Personification is a type of metaphor in which things that are not human are given human attributes.
"The sun was smiling"
"The machine gun screamed"
"The trees swayed contentedly"
An idiom is an expression with a meaning that cannot be understood from the literal meanings of the individual words.
Idioms, like dead metaphors, are incredibly common in every day use.
"Things are getting out of hand"
"It was a piece of cake"
"He'll do it when pigs fly"
All of the above are idioms, because the meanings of the words don't really have anything to do with the meanings of the expressions. For example, "It was a piece of cake" means "it was easy" - but what do pieces of cake have to do with easiness!?
Oxymorons & Antithesis
An oxymoron is a figure of speech in which words of contradictory meaning are collocated (paired or grouped).
"It had a bitter sweet ending"
"Sometimes you have to be cruel to be kind"
"It's a new classic"
Similarly, antithesis involves the juxtaposition of two contrasting ideas in close proximity to one another.
Neil Armstrong used antithesis in his famous quote "one small step for man, one giant leap for mankind"
The first line of 'A Tale of Two Cities' by Charles Dickens also employs antithesis: "It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness..."