A sonnet is a form of poetic writing. A sonnet has fourteen lines (in English) and ten syllables to each line.
Different rhyming patterns have been followed at different times. The famous English playwright William Shakespeare wrote 153 sonnets and generally ended them with a rhyming couplet.
A sonnet is almost always a complete poem and not one that can be set to music, like a ballad or lyric.
One famous sonnet used was in Romeo and Juliet, when the lovers meet for the first time and Shakespeare divided a sonnet between them.