A poem composed of 14 lines.
[Reads dictionary explanation, yawns: not fully attentive]
The English convention is 10 syllables for each of these, and a choice of styles: Petrarchan, Shakespearian or Miltonic being our main three. They differ in rhyme scheme and pacings of octaves (first eight lines) and sestets (last six lines, aka sextets.) And it should be Iambic (the rhythm that runs soft LOUD soft LOUD.)
[Nods head absentmindedly: facts are read with some recall, except:]
'Why these particular numbers?'
[Scratches head to denote thought]
They are pleasantly even
They fit musical forms (sonnet from the Italian, 'little song')
They are long enough to set up and answer a question or two, not so long the reader loses track/interest
Long enough to play with form and make different styles from one format: a sort of literary franchise?
It proved popular, so writers kept at it
The Shakespearian form breaks mostly into 3 quatrains (4 line stanzas) followed by 1 couplet (2 lines). The first 3 quatrains set up a surprise in the final two lines.
[Considers writing in this format. Pictures how the current pile of projects is stacking up]
'UnwielDY? Is that Iambic?'
[Sighs. Eats a sandwich]