A famous poetic form. There's many different variations, but typically in English it is fourteen lines of iambic pentameter. They tend to be devotional love poems but aren't necessarily. They tend to have a turn (or volta to be fancy) within them

Shakespeare wrote a whole load of them:

Sonnet 130

My mistress' eyes are nothing like the sun;
Coral is far more red than her lips' red;
If snow be white, why then her breasts are dun;
If hairs be wires, black wires grow on her head.
I have seen roses damasked, red and white,
But no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
I love to hear her speak, yet well I know
That music hath a far more pleasing sound;
I grant I never saw a goddess go;
My mistress, when she walks, treads on the ground.
And yet, by heaven, I think my love as rare
As any she belied with false compare.

The rhyme scheme here is ababcdcdefefgg, with alternating rhymes followed by a rhyming couplet. You can see the volta in the last two lines- the turn away from insults and into praise.