One of the main things that distinguished poetry from prose is that phrases are broken up into lines. Due to this breakage, a sort of spacing or form emerges. More than that even, poets focus on spacing, in general, a lot.

We should discuss the way in which the spacing works for the poem so we can see why poets might do this. If you disagree with or want to add something, please do. As always, I would love a discussion.

I'll try to list the different uses as they come to me, so the following is not sorted in any particular order:

Training

There's this idea that we've been trained, via English classes, etc., to recognize a poem by its spacing. According to this reasoning, when we see the breakage in lines, we immediately (and perhaps subconsciously) realize that we should be thinking in terms of metaphor or in terms of something other than prose. I don't think this facet is all that useful to delve into so I wont.

Breaks the Pattern of Reading

When we read prose, we tend to read in word clusters rather than individual words. By this, I mean that our eyes fall on several words at a time, and we ascertain their meanings together. This all happens very quickly, and we do it because it allows us to scan a page efficiently. It would be pretty excruciating if we, as readers, paused to examine the possible meanings of every single word presented in a novel or short story.

The lines of poetry prevent us from reading this way. We can no longer scan left-to-right in the same way. By preventing us from scanning, the lines draw attention to each word used. Because of this, poetry is often a lot more language-intense than prose. It focuses on the nature of words and the different effects that each word produces in the mind of the reader.

Visual Effect: Shape

The line breaks also allow the poet to fit the poem into a shape. This is pretty straight forward. For example, a poem might describe a pyramid, and to add another layer to the poem, the lines are shaped in the style of a pyramid, with short lines at the top and longer lines at the bottom. I don't know how useful this device is at achieving poetic goals. Shape is definitely one of the less-popular devices used.

Visual Effect: Emptiness

Sometimes, poets choose to insert extra spacing between words or stanzas. Due to the nature of spacing (and space, in general), this can add to the meaning of the poem. It can place emphasis on (a) certain word(s) and set the word(s) apart from the rest of the poem. Space is also empty, and it suggests a nothingness. Thus, it is good at conjuring a sense of loneliness. In contrast, if you crowd the words together, it creates an altogether different effect, and if you do the two at different parts of the poem, you get a strange visual effect.

I don't know if this has been done a lot in poetry a lot though. It could be interesting to experiment with. I think that would bring poetry closer to the art of drawing or painting in that the words would form an image against the space, and that image would hold some sort of meaning. This is pretty similar to "Visual Effects: Shape", but its more vague, so I included it in a different category.

Enjambment

This is the really common one that lots of poets use. When you cut a sentence with a line-break, its called enjambment. Enjambment can affect the sound or flow of the sentence, and it can affect the meaning of a sentence. For example, here's something from Anne Carson's "Autobiography of Red":

He thought softly of other words he could keep with him like beach and screach. Then they moved Geryon into his brother's room

The breakage of the line "Then they moved Geryon into his brother's room" creates enjambment, and the enjambment plays with the meaning of the word "they". Depending on which context you look at in, "they" can take on different meanings. For example, if you ignore the last line, "they" refers to the words that Geryon was thinking about ("beach" and "screach") because those are the last nouns referenced. However, if you include the last line, then "they" refers to Geryon's parents. Both readings are legitimate, and this double meaning adds something to the poem.

Sound

When reading out loud, we tend to pause at the ends of lines. This can add (or disrupt) a flow.

Others?

Those are the ones I can think of at this moment. There must be more, but I can't recall right now. If you guys remember, feel free to add.

quick question to make sure I don't get too unfocused. D you want to talk about just line breaks, or about spacing in poetry in general?

Spacing in general - spacing between words as well as spacing between lines as well as spacing between stanzas. I'm not sure what other kinds of spacing poets can employ.