Placing two like-sounding words together, typically at the end of a line, creates rhyme. When poetry scholars talk about rhyme they are generally referring to a rhyme scheme and map it out with letters. For example, if a poem is four lines long and every other line rhymes then the rhyme scheme would be "abab."
Rhyming is typically used to show when lines break, but it can also be used to show how words fit together. Most of the time, words that rhyme will somehow be connected.
Some poets, particularly free verse poets, like to use off rhyme where the words sound similar but not exactly alike. For example, cow and plow would be examples of explicit rhymes, but blood and cold would be off rhyme.
There are two types of explicit rhyming:
Assonance: when the vowel sounds match. For example, trim, dim or him.
Alliteration: when the consonant sounds match. For example, fresh, fire coal, falls.
Poem Construction
Poems are typically written in lines, and these lines are placed together in various ways to form the body of the poem.
If only two lines are put together, the poem is said to be written in couplets.
If three or more lines are put together, the poem is said to be written in stanzas, which can be colloquially referred to as poem paragraphs.
When a series of stanzas are made up of regular lines, for example six four-line stanzas, the stanzas are said to be isometric. If the stanzas are made up of irregular lines, for example three four-line stanzas followed by one three-line stanza, the stanzas are said to be heterometric.