Lyrics are the written words in a song. Lyrics are usually written during or after the underlying music is composed. However, it is possible that music is added after the lyrics are written.
From the Greek, a lyric is a song sung with a lyre. (A harp used by ancient Greeks for accompaniment.) Now, it is commonly used to mean a song of no defined length or structure. A lyric poem is one that expresses a subjective, personal point of view.