Denotation: The actual dictionary definition of a word
Connotation Emotional and imaginative association surrounding a word; the meaning given to a word through its use in a society or culture
For example: Annette was surprised Annette was amazed. Annette was astonished. The words are synonyms (denotations), but they have slightly different connotations.
Synonyms can often have a positive, neutral, or negative connotation. For example, the synonyms scrawny and slim have similar denotations, (thin or not having much weight) but most people would rather be called slim than scrawny.