The Souls in Purgatory are the souls of those who died in the state of grace (i.e. as God's friends), but who are not yet cleansed enough from sin to enter Heaven. When we are sorry for our sins and confess them, God joyfully forgives us. However, each sin wounds our soul, and the damage remains even after it is forgiven. Souls who neglect to repair this spiritual damage, by doing penance, are not ready for Heaven when they die, even if they don't deserve to go to hell. So God in His mercy allows them to spend time suffering in Purgatory, in atonement for their sins before entering Heaven. The Souls in Purgatory are part of the Communion of Saints (they are sometimes referred to as "The Church Suffering"), and we have the obligation as Catholics to pray for their speedy release from Purgatory.
So Purgatory is a state of purification, where the soul can be cleansed of all the sins and damage from sin which hinder it from reaching Heaven. The fact that Purgatory exists is a proof of God's overwhelming mercy. Souls who die without having taken care to properly prepare their souls for Heaven, would actually prefer to suffer in Purgatory rather than have to enter the presence of God and stand among the glorious saints and angels, stained with even the slightest speck of sin!
Imagine if you were invited to a party, and when you arrived, having not bothered to shower and change from your old clothes, you discovered that the room was full of glittering, gorgeously dressed kings and queens! Imagine how ashamed you would feel, and how glad you would be if someone took you aside and showed you a room where you could get suitably washed and dressed.
However, Purgatory is a place of suffering, as the saints saw, when Souls in Purgatory appeared to them wrapped in flames and begging for prayers. We know, of course, that souls are not material, so the vision of fire is certainly only a symbol of sufferings which we don't understand. Part of the suffering of Purgatory, of course, is that the souls there can't see God, and they long with all their hearts to be with Him. Perhaps another part of the suffering is seeing the awful damage which your sins have caused to others - following the results of your sinful actions as they continue to affect people centuries later. Who knows? (Just picture how horrible it would be to have to listen to ALL of the conversations you ever had during your whole life, only now intensely aware of every unkind, idle, foolish, or unchaste word, and seeing the effect of those conversations on the souls around you!)
How can we help the Holy Souls?
It's true that we often entirely neglect to pray for the poor Souls in Purgatory. We want to help those who suffer in our world, whether human or animal, but sometimes we forget about helping the Souls in Purgatory, since we can't see them. However, there are many different ways in which we can help them and earn their eternal gratitude:
Have Masses said for them. Some Souls are so close to Heaven that all they need are the graces of ONE MASS to set them free. And we can do this for them so easily!
Say prayers for them. On this site we have collected a few prayers for the soul in purgatory but you can offer any prayer for them, such as the Rosary or the Chaplet of Mercy.
Offer sacrifices for them. When you do something kind for someone, or you make a sacrifice, remember to say inside, "Jesus, this is for the Souls in Purgatory." Or, you can pick specific days, and decide that you will offer everything good you do that day, for the relief of the Souls in Purgatory.
Gain indulgences for them. By gaining indulgences for the Souls, you can give them enormous graces from the "treasure-house of the Church," by saying just one little prayer or doing a simple little action.
How do we avoid Purgatory?
It is possible to avoid Purgatory and go straight to Heaven after your death!
Frequent Communion and Confession are important means to purify our souls and keep us at all times ready for death.
When we are sorry for our sins and go to Confession, God completely forgives us. However, each sin wounds our soul, and the spiritual damage we have caused remains even after our sins are forgiven.
We can repair this damage to our souls by our prayers and sacrifices, and by perfect contrition, being perfectly sorry for our sins because of the sorrow they cause God, not just because we know we will be punished for them.
You don't have to do all the work by yourself! Why not ask God to help you with the merits and sufferings of Jesus? Tell Him you want to go straight to Heaven when you die, to be with Him, and He will be glad to help you, because He wants you to be with Him more than you can imagine!
And of course, all this is made easier if you have a true desire to serve God and become a saint. By your love for God, you can atone for your sins so perfectly that you will be worthy to go straight to Heaven when you die! Hey, that's just one more reason for becoming a saint!