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. 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.