"and wrote page after page of villanelles and sonnets"

A villanelle is a complex 19-line poetic form which follows strict rules regarding rhyme and the repetition of certain lines;

A sonnet is a 14-line poem which follows prescribed rules regarding rhyme and metre.

Listen to Dylan Thomas, said to be one of Sylvia Plath's favourite poets, reading his villanelle  'Do Not Go Gentle into That Good Night':