Synonyms are words that have the same meaning as each other. Small/little, fast/quick. Antonyms are words that mean the opposite of each other. Big/small, fast/slow.
A synonym is a word which means the same (or nearly the same) as another word. For example: shout is a synonym for yell. Polite is a synonym for courteous.
An antonym is a word which means the opposite of another word. For example: tall is an antonym for short. Smart is an antonym for