Who it's for
The Catechism's wording is the best place to begin: people who die in God's grace and friendship, but still imperfectly purified (CCC 1030). That covers more people than you might think.
The largest group is ordinary believers who die with venial sins on their souls. The Church distinguishes mortal sin, which breaks our friendship with God, from venial sin, which wounds it without destroying it. Mortal sin needs three things at once: grave matter, full knowledge and deliberate consent (CCC 1857). Venial sin is everything lighter: the sharp word, the white lie, the prayer said on autopilot, the small vanity. It doesn't cut us off from God, but it weakens love, and most of us will die with a bit of it still there, perhaps never noticed.
A second group is people whose sins have been forgiven, even grave ones, but whose lives were shaped by them. Picture someone who lived selfishly for decades, repented honestly near the end, received the sacraments and died at peace. The guilt is gone. The habits of a lifetime are not. The Council of Lyons spoke of this exactly: those who die truly repentant in charity, but before making full satisfaction.
A third group is people who never knew Christ fully but sought God sincerely. The Second Vatican Council taught that those who, through no fault of their own, don't know the Gospel but honestly seek God and try to do His will under the influence of grace can be saved (Lumen Gentium 16; CCC 847). The Church doesn't tell us how that happens in any given case. But if they're saved, it's reasonable to think that they, too, would need to be made ready for the full vision of God.
Who it isn't for
Purgatory isn't for those who die in unrepented mortal sin, having turned from God for good. It is never offered as a second chance. It belongs only to those who are already His.
And some people don't pass through it at all. The martyrs, who give their lives for Christ, have always been honored as entering Heaven directly, and so have the great saints, whose union with God was so complete by the end that nothing was left to purify. That's why the Church is so confident asking for their prayers. Someone who dies right after Baptism also goes straight to Heaven, because Baptism takes away all sin and all the punishment due to sin (CCC 1263).
The hard cases
These come up more than almost anything else on this subject, and they deserve gentleness.
About infants who die without Baptism: the Church has no definitive teaching. She entrusts them to the mercy of God and expresses hope that there's a way of salvation for them (CCC 1261). It's a place for hope, not despair.
About those who take their own lives: the Catechism says we shouldn't despair of their salvation. Grave psychological disturbance, anguish and fear can lessen a person's responsibility, and God can provide, in ways known only to Him, a chance of repentance (CCC 2282-2283). The Church prays for them, and so should we.
About someone who left the faith or seemed to lose it: only God sees the whole of a human heart. We don't, and we're asked not to pretend we do.
Can we know who is there?
No, and that's worth sitting with. The Church canonizes saints, which is to say she declares with certainty that they're in Heaven. She has never declared that any named person is in Purgatory, and she has never declared that any named person is in Hell. The dead are known to God alone.
That leaves us two things to do. Don't stop praying for someone because you feel sure they're already in Heaven. If they don't need the prayer, God can apply it to someone who does, and nothing offered in love is lost. And don't assume the worst about anyone either. At a funeral, nobody should be standing there wondering whether the person was lost. We trust in His mercy, and we pray.
And you?
Will you go to Purgatory? Probably, if you're like most of us. Most people who love God sincerely will die with something left to heal. That isn't meant as a threat. It's an invitation to start now, to let God work on the tangles through Confession, forgiveness, patience, prayer and small acts of mercy.
And whatever happens, remember who Purgatory is for. It's for those God has already claimed.
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