The villanelle is a French verse form which became popular in English in the late nineteenth century. By the time Dylan Thomas was writing in the 1940s, it was regarded as an old-fashioned form, primarily associated with the aesthetic movement more than fifty years before. Thomas was instrumental in reviving the form, and "Do Not Go Gentle into That Good Night" is now probably the most famous villanelle in the English language.
Two attributes of the villanelle form make it perfect for this poem: insistence and inevitability. The line "Do not go gentle into that good night" is a magnificent and memorable one. The villanelle form allows it to appear four times in a nineteen-line poem, as does the striking phrase, "the dying of the light." These phrases are repeated insistently at the end of every stanza. This repetition creates a sense of inevitability. Once you have read the first stanza of a villanelle, you know how every other stanza is going to end. The inevitability of death is mirrored in the repetition and the rhyme scheme of the villanelle form. Tension is created within the poem by the struggle against the inevitable, and this conflict is all the more powerful for being tightly constrained by the form.