A Songwriter, Composer and a Lyricist make up the types of contributors for a song that were expressly involved in the creation of a song's composition, whether that be the melody or the lyrics or both.
All songs require at least one Songwriter or Composer.
Here is a short list of each role and their generally understood type of contribution to a song:
Songwriter: Normally the person that creates a song melody along with the song's lyrics.
Composer: Normally the person that creates only a song's melody.
Lyricist: The person that creates only the lyrics for a song.
To learn more about Songwriters/Composers click here.