Who are the Church's "cloud of witnesses"?

The Church is not only composed of people here on earth, but also a "cloud of witnesses" who intercede for the Church in Heaven.

When describing the Church’s members currently in Heaven, they are sometimes called a “cloud of witnesses.”

This phrase is used by St. Paul in his letter to the Hebrews:

Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with perseverance the race that is set before us.
Hebrew 12:1

The Catechism of the Catholic Church refers to this phrase in its section on prayer:

The witnesses who have preceded us into the kingdom, especially those whom the Church recognizes as saints, share in the living tradition of prayer by the example of their lives, the transmission of their writings, and their prayer today. They contemplate God, praise him and constantly care for those whom they have left on earth.
CCC 2683

These witnesses include saints whom the Church officially recognizes, as well as anyone else who has “run the race” and now resides in Heaven.

The Catechism stresses in particular their “job” of interceding for all of us on earth:

When they entered into the joy of their Master, they were “put in charge of many things.” Their intercession is their most exalted service to God’s plan. We can and should ask them to intercede for us and for the whole world.
CCC 2683

The good news is that we are not alone in this world and that we have a “cloud of witnesses” cheering us on, praying for us that we will “run the race” and join them some day in Heaven.

Who are the Church’s “cloud of witnesses”?