There are different types of sonnet - a popular type, and the one Donne used, is called the Petrarchan sonnet form. This form defined a sonnet to be composed of fourteen lines only, divided into two distinct parts. The first 8 lines are called the Octet, and present a "problem, idea, or situation", the following six lines are called the Sestet , and present an answer or some sort of comment on the problem presented in the octave.
Sonnets also had a standard "meter". Explaining meters in detail is beyond the scope of this
article, but in brief, Petrarchan sonnets used iambic pentameter, which was a
very popular meter and the one Shakespeare used in his plays. "iambic pentameter" means "alternating
stress on the words, five stresses per line". Examine the following Line from "Oh my black Soule!
now thou art summoned", a poem in iambic pentameter by John Donne, with the stresses marked in
Oh make thy selfe with holy mourning blacke;
This clearly illustrates the way the stresses alternate on the line.
Donne also employed enjambment, the technique of running one line into another which adds to the emotional effect of a poem, in the octet. Sometimes the metre is not simple iambic pentameter, although it generally is.
Donne's sonnets are sometimes described as "rough" for these reasons, but Donne had a clear purpose every time he "broke the rules" of strict sonnet form - it adds to the emotional intensity of his argument and makes it stronger.