This is a broad question. It will certainly elicit many different responses. Style in poetry can refers to almost anything. It can refer to the way an authors uses meter. For example, if an author always uses dactylic hexameter, then it might be his style. The same can be said if a poet uses literary devices such as personification, alliteration, assonance. Style can also be non-technical. It can refer to content. If an author always uses imageries from nature, then it can be his or her style. In short, style can refer to almost any aspect of poetry as long as the author tends to do it often.
Style in poetry involves the method which a poet uses to convey meaning, tone, and emotion in his/her poem. For instance, the meaning and significance can be conveyed through the form of a poem. Certainly, an ode or a sonnet is a more formal arrangement used more for a serious subject (unless the poet is being satirical). Musical devices such as rhythm and rhyme are part of a poet's style. Words, too, are often chosen for sound as well as for meaning. An essential element is repetition which reinforces meaning while variation invites interest. In fact, all things people enjoy have these two elements.
The arrangement of the words in a line, as well as the arrangement of lines in the entire poem both contribute to a poet's style. Emily Dickinson, for instance, makes uses of dashes and capitalization as a stylistic device while e.e.cummings never capitalizes any word. These devices are used because poets wish to bring a sense and a perception of life, widening peoples' contacts with existence. And, since poets' concern are with experience, they choose certain stylistic devices to create significant and new experiences for their readers. In poetry, experiences can be synthesized, analyzed, enjoyed simply, etc. But, the way in which poets present their poems is their style.
Other stylistic devices that poets use are connotation and denotation, the suggested as well as the literal meanings of words. Figurative language such as metaphors and similies also add more meaning and picturesque language to a phrase or word as well as creating speed to the line, for example, as alliteration does.
Style can refer to a lot of things in poetry. It can be referring to the meter (free verse, haiku, Damante, blank verse, etc.), it can be used to refer to the technique used in poetry (using oonomatopoeia, alliteration, assonance, simile, metaphor, etc.) and it can be used to describe the general style a particular poet is known for.
The style a poet is known for refers to the poet's typical use of meter and techique as well as subject matter. It can also be used to refer to a particular era of poets where the poets all used a similiar style that was popular for their time period, such as The Romantic Age (William Wordsworth and William Blake) or the Beat Poets (Jack Kerouac and Allen Ginsberg).