They are part of the Communion of Saints. The Communion of Saints is comprised of the church militant (those alive on earth), **the church penitent (those undergoing purification in purgatory in preparation for heaven), **and the church triumphant (those already in heaven). (The damned are not a part of the Communion of Saints.)
Souls in Purgatory are also called Holy Souls. They are souls of those who have died in the state of grace but who are not yet free from all punishment due to their unforgiven venial sins and all other sins already forgiven (for which satisfaction is still to be made). They are certain to enter Heaven but must first suffer proportionately in Purgatory. It is the teaching of the Church that these souls are aided by our prayers and good works, and especially by the Sacrifice of the Mass. We can pray and ask for their intercession. We must pray for them too. Remember, once they make it to Heaven and become Saints in Heaven they will remember us and our help and prayers for them. They will remember us and will most likely pray for us too. It’s always good to have friends in Heaven.