What is the term for a 14-line love poem that usually ends with surprising idea?

A 14-line love poem that usually ends with a surprising idea is called a sonnet.

While sonnets don't *always* end with a surprising idea, it's a common characteristic, especially in the Shakespearean sonnet. This type of sonnet has a specific rhyme scheme (ABAB CDCD EFEF GG) and often uses a "volta" or turn in the final couplet to introduce a twist, a revelation, or a surprising insight into the subject of the poem.