What's a composer?

well  they're both composers such as; Rogers ands Hammerstein; Lerner and Lowe,

For a musical, usually, the composer writes the music while the lyrics are written by the librettist.

Generally in a musical the “composer” writes the music and the lyrics (words) are written by a “lyricist”. However, the term “composer” could be used to describe anyone who contributes to writing a song - words or lyrics.

In a more general sense, a “composer” could write the music, the lyrics or both.

A composer, lyricist and librettist, in different contexts, might all write lyrics.

 curtisports2 is wrong.  A composer can write anything; music, poetry.  A lyricist is just a composer that doesn't compose the music.

Kathy knows.

A composer composes the music. A lyricist write the words.

Charles, the question was specific to the musical, not composers in general.

The words to the music are written by the librettist.

The composer creates the tune, the melody.  The lyricist writes the words the singer sings.

They can be both of them.