Diction refers to a writer's or speaker's choice of words and style of expression. It has two main meanings:
1) Distinctive vocabulary choices that establish tone and characterization. For example, using many verbs of physical movement suggests an active character.
2) Pronunciation and clarity of speech. This is more precisely called enunciation or articulation.
Diction has multiple aspects, including register (formal vs informal words), word choice, syntax, and eight main elements: phoneme, syllable, conjunction, connective, noun, verb, inflection, and utterance. Imagery refers to the mental pictures or "images" created by descriptive language in poetry or prose.