The words and phrases a poet chooses to use are referred to as B. diciton. In other words, diction is the language that a poet chooses to employ in the poem.
Theme is the main topic of the poem. A poem may also have various themes, such as love, friendship, betrayal, etc.
The structure of the poem refers to the way the poem is written in stanzas and lines, having a rhythm, etc.
Meter refers to a way of constructing rhythm, so it is the pattern of the stressed or unstressed syllables.