collection of Christian or other religious hymns, usually in the form of a book

A hymnal is a collection of hymns. All the people ("the congregation") usually sing some of these hymns, during a Christian religious service. A hymnal is different form a song book; each group has its own hymnal. Groups who aren't religious also have hymnals: there's one for scouts, there are hymals for different student associations, or for soldiers.

Bookshelf with hymnals, in a church in Erfurt.
Hymnals in a church in Norway.
A Mennonite church, with hymnals.

Hymnals usually contain the text of the hymn, and sometimes there are scores (which render the melody). For hymns that are well-known, the score may be missing.