A Sonnet is a poem consisting of 14 lines (iambic pentameter) with a particular rhyming scheme:
Examples of a rhyming scheme:
#1) abab cdcd efef gg
#2) abba cddc effe gg
#3) abba abba cdcd cd
A Shakespearean (English) sonnet has three quatrains and a couplet, and rhymes abab cdcd efef gg.
An Italian sonnet is composed of an octave, rhyming abbaabba, and a sestet, rhyming cdecde or cdcdcd, or in some variant pattern, but with no closing couplet.
Usually, English and Italian Sonnets have 10 syllables per line, but Italian Sonnets can also have 11 syllables per line.
French sonnets follow in this same pattern, but normally have 12 syllables per line.
Song
A Song is an expression of a poet's personal emotions, meant to be sung. Lyrics in a song contain verses (lines that make up a song; sung poem) and a chorus (a repeating verse in a song (refrain).