A line is a subdivision of a poem, specifically a group of words arranged into a row that ends for a reason other than the right-hand margin. This reason could be that the lines are arranged to have a certain number of syllables, a certain number of stresses, or of metrical feet; it could be that they are arranged so that they rhyme, whether they be of equal length or not. But it is important to remember that the poet has chosen to make the line a certain length, or to make the line-break at a certain point. This line-break, where a reader has to turn back to the start of the next line, was known in Latin as the versus, which translates as "turn", and is where the modern English term "verse" comes from. It is one of the strongest points of a line, which means that words that fall at the end of a line seem more important to a reader (an effect that rhyme can intensify); other strong points are the start of a line, and either side of a caesura.
Just about every poem in the Archive has lines (prose poems can be argued over - they either don't have lines, or have really long lines). It has been suggested that a line is supposed to be the length of a breath, so that a long line should leave you breathless, or a short line should make you feel like you're hyperventilating. This reason may be debatable, but an alertness to the frequency of the line-endings is part of reading poetry.
Poets will sometimes use a regular line length, which, naturally, gives a sense of regularity; they may use different line-lengths for every line, which suggests that each line is set to be the length it is by its content; or they may use different-length lines that occur in a repeated pattern, which has elements of both. ('Length' can be measured in a few different ways - see 'Metre' for more on this.) Examples of each are, respectively, Anthony Thwaite's 'Simple Poem', Adrienne Rich's 'Fox', and Robert Minhinnick's 'The Yellow Palm'.
Ciaran Carson's 'Belfast Confetti' has some of the longest lines in the Archive; it gives the poet freedoms within the line that Charles Tomlinson doesn't allow himself in 'A Given Grace', but the short lines of the latter poem make sense with its presentation of a close-up examination, which would, perhaps, jar with Carson's presentation of a violent event and wanderings through a city.