Accentual verse has a fixed number of stresses per line or stanza regardless of the number of syllables that are present. It is common in languages that are stress timed such as English as opposed to syllabic verse, which is common in syllable timed languages such as classical Latin.
Accentual verse derives its musical qualities by the alternation of stressed and unstressed syllables in more or less regular patterns, as in this example: to be or not to be (bold represents stressed syllables). This is an example of the iamb, the metrical foot that is most commonly used in English-language poetry.
Another common effect is to alternate lines of different syllable count, or length. For example, in this verse from the ballad Barbara Allen, the first and third lines have four stresses each while the second and fourth have three.
In Scarlet towne, where I was borne,
There was a faire maid dwellin,
Made every youth crye wel-awaye !
Her name was Barbara Allen.